Is a National Insurance number personal data? How to redact it

A National Insurance number is personal data, the AI never needs it, and it takes seconds to replace with a placeholder that keeps the question intact:

NI number: QQ 12 34 56 C   →   NI number: [REFERENCE_1]

Redact a National Insurance number in seconds →

We use the government’s own specimen number, the one the GOV.UK Design System prints, so that nobody’s real number appears on this page; a real one after the same label comes back as [NINO_1]. Either way the model still knows a National Insurance number was there, so a question about a payslip, a pension letter or a P60 reads exactly as before.

The government’s own advice points the same way. GOV.UK tells people, in plain words, not to share their National Insurance number with anyone who does not need it, and then lists the organisations that may need it. No AI tool is on that list.

This guide covers what the law says about the number, what HMRC says about its shape, where it hides in the files an office handles every day, how to take it out by hand, and what the Nonimo app does with it. It also covers the details that sit beside the number on a payslip, which matter just as much once the number has gone.

Is a National Insurance number personal data? Yes, and the ICO names it

Article 4(1) of the UK GDPR defines personal data as information relating to an identified or identifiable natural person, and it gives a short list of identifiers that make someone identifiable. The second item on that list is an identification number. Section 3 of the Data Protection Act 2018 gives the same definition for a living individual, with an identification number again on its list of identifiers.

The ICO goes one step further and names the number outright. In its guidance on indirect identification, the national insurance number appears in a short list of information that allows an individual to be singled out from others, next to a car registration and a passport number.

So the question in the title has a clean answer, and it is the same in a payroll bureau in Leeds as in a solicitor’s office in Cardiff. What follows from it for the AI tools themselves is in our guide to GDPR compliant AI tools in the UK.

Not special category data, and no Article 87 in the UK

Article 9 lists the categories that carry extra protection: racial or ethnic origin, political opinions, religious or philosophical beliefs, trade union membership, genetic data, biometric data used to identify someone, health, and sex life or sexual orientation. A National Insurance number is none of those. A payslip line next to it can still reveal one, which is the subject of our guide to special category data and AI.

The GDPR as originally written has an Article 87 that lets each country set its own conditions for processing a national identification number. The UK version no longer does: legislation.gov.uk shows Article 87 as omitted from 31 December 2020. In the UK, the protection for an NI number comes from the general rules.

2020

That makes the British position simple to explain to a team. There is no list of bodies entitled to the number written into data protection law, and no extra condition to check before you hold it. There is one question, the same one you ask of every other field in a file: does this task need it?

Minimisation is the rule that does the work

The general rule that matters most here is data minimisation. Article 5(1)(c) requires personal data to be adequate, relevant and limited to what is necessary for the purpose. When the purpose is a question to an AI tool about why someone’s pay went down, the number adds nothing to the answer, so the cleanest way to meet that principle is simply to leave it out.

That is a comfortable position to be in. You are not applying a complicated balancing test, you are removing one field that the task never needed.

What a National Insurance number looks like, according to HMRC

HMRC’s National Insurance Manual sets out the format at NIM39110: two letters, six numbers and a final letter, which is always A, B, C or D. It prints QQ 12 34 56 A as its example, with the note that the example should not be used as an actual number.

2letters in the prefix

6digits in the middle

1suffix letter, A, B, C or D

The same page explains which prefixes never appear. The letters D, F, I, Q, U and V are not used as either letter of a prefix, O is not used as the second letter, and the prefixes BG, GB, KN, NK, NT, TN and ZZ are not used. That is why the QQ in HMRC’s example is safe to print: no real prefix contains a Q.

HMRC and DWP have also used reference numbers that look like National Insurance numbers without being one. The manual names OO for some tax credit cases, FY and NC from older schemes, and PZ and PY, which the old Inland Revenue used for tax only cases until August 2002. A good detector tells these apart, and Nonimo does: after the label NI number, they come back as a general reference rather than as [NINO_1].

Spaced, unspaced and in a column of its own

GOV.UK writes the number without spaces, as QQ123456B, on its page about your National Insurance number. HMRC’s manual writes it with spaces. Real paperwork does both, and a search for one form will not find the other.

In a spreadsheet the number usually has a column to itself, headed NINO, NI No or National Insurance. That makes it the easiest thing on the sheet to remove before anything is shared, and our guide to redacting an Excel spreadsheet shows how to check the hidden columns as well as the visible ones.

Where the number hides in an ordinary office file

GOV.UK’s page on finding your National Insurance number lists where to check first: a payslip, a P60, and letters about tax, pensions or benefits. Those are exactly the documents an office forwards, summarises and asks questions about every week.

document	who usually holds it	where the number sits
Payslip	payroll bureau, HR, the employee	header, beside the name and tax code
P60 and P45	accountant, payroll, new employer	employee details box
Pension letter	adviser, solicitor in a divorce	reference line or first paragraph
Benefit letter	council, adviser, family solicitor	reference line
Starter details	HR, payroll bureau	a field of its own
Payroll or HR export	anyone who opens the file	its own column

GOV.UK for the payslip, the P60 and the letters; the rest from ordinary office practice.

The pattern is the same in every row: the number is in the part of the page that identifies the person, and the question you want answered is somewhere else. That separation makes it simple to take out.

The CSV nobody looked at

A payroll export is the document most likely to carry hundreds of National Insurance numbers at once. It rarely gets opened in full before it is attached, because the person sending it only needs one column from it. Before a CSV goes anywhere near a chatbot, read our guide on how to upload a CSV to ChatGPT without personal data, which covers deleting whole columns rather than editing cells.

What GOV.UK says about sharing the number

The government’s advice on keeping the number safe is short and direct. On the page about your National Insurance number, under the heading about keeping it safe, it says that to prevent identity fraud you should not share your National Insurance number with anyone who does not need it.

It then lists the organisations that may need to know it: HMRC, your employer, the Department for Work and Pensions, your local council if you claim Housing Benefit, Electoral Registration Officers, the Student Loans Company, your pension provider, your ISA provider, authorised investment providers, platforms you sell through and cryptoasset services where they report to HMRC, and Veterans UK. That is 12 entries in all.

12

The same caution shows in how HMRC hands the number out. GOV.UK says HMRC cannot give you your National Insurance number over the phone or on webchat. You download a letter from the HMRC app or your personal tax account, or it arrives by post, usually shortly before your sixteenth birthday.

The question for an office, rather than for an individual

That advice is written for the person who owns the number. An office holding hundreds of them is in a stronger position than an individual and a slightly harder one at once: stronger because it has a lawful reason to hold them, harder because each one belongs to someone who trusted it to keep that reason narrow.

The reason an employer or a bureau holds the number is payroll and reporting to HMRC. Asking an AI tool to explain a tax code, draft a letter or check a calculation is a different purpose, and it works just as well without the number. What each provider does with the text you send is in our guide to whether ChatGPT shares your data in the UK.

What the ICO saw in August 2026

On 7 August 2026 the ICO issued a reprimand to ACRO, the national police unit that provides police certificates, over security failings behind three compromises of its customer portal between 2021 and 2023. Among the data staged for extraction were identification numbers, and the reprimand names them: national insurance number, driving licence numbers and passport details. At most 10,920 people had data staged, ACRO notified 84,048 as a precaution, and 35 complaints were evidenced to the ICO.

10,920people at most whose data was staged for extraction

84,048people notified as a precaution

35complaints evidenced to the ICO

The complaints described concern about identity theft and financial loss. That is the reason GOV.UK gives for keeping the number close, seen from the other side, and the reason a payroll query is better off with a placeholder in it.

Why the AI never needs the number

Think about the questions an office actually asks an AI tool in a normal week. Every one of them has the same answer with the number as without it:

the question	what the answer depends on	does it need the NI number?
Why did her net pay drop in September?	gross pay, deductions, tax code	no
Is this tax code right for a second job?	the code and the two incomes	no
Summarise this pension statement	the figures and the scheme rules	no
Draft a reply to this benefit letter	what the letter asks for	no

Invented examples of everyday office questions.

A National Insurance number exists, in GOV.UK’s words, to make sure contributions and tax are recorded against one person’s name only. The model cannot look it up, cannot check it and has no use for it, so the only thing the number does in a prompt is travel.

The answer comes back just as good

People sometimes worry that a prompt full of placeholders gets a vaguer answer. The placeholder keeps the one thing the model can use, which is what kind of detail sat there. A line that still says NI number before the placeholder tells it the payslip carries a National Insurance number, so it can still suggest checking that the number matches the one on the P60. It simply cannot repeat the number back, which is the point.

What to keep instead

Keep the facts the question turns on, written as roles: the employee, the employer, the pension scheme. Keep the tax code if the question is about tax, because a code such as 1257L describes an allowance rather than a person. Keep amounts if the question needs them.

That is also how the Nonimo app works. In our test below, the tax code came back exactly as it went in, because it does not identify anyone, while the name, the date of birth, the email address and the NI number were replaced. Accountants who want to see that on a client file can start with our page for accountants.

How to redact a National Insurance number by hand

For a single document and a single question, doing it by hand is quick. The order matters, because the commonest mistake is finding one form of the number and missing the other.

1. Work on a copy. Never edit the file that goes back into the client record.
2. Search for the label. Look for National Insurance, NI number, NI No and NINO, and read the line each one sits on.
3. Search for the shape. Two letters followed by six digits, with and without spaces, catches the numbers that were typed without a label.
4. Replace, do not delete. Write a placeholder such as [NI number] so the sentence still reads, and use the same one every time for the same person.
5. Check the headers, footers and file name. Some payroll templates print the number on every page, and a file name can carry it too.
6. Read it once more as a stranger would. If a colleague could still tell who it is, something else on the page needs to go.

For step 3, any editor with a regular expression search will do the work, including LibreOffice, Notepad++ and Visual Studio Code. This pattern finds the number with or without spaces:

[A-Z]{2} ?[0-9]{2} ?[0-9]{2} ?[0-9]{2} ?[A-D]

In a spreadsheet, replace the whole column

If the numbers are in a column, there is no reason to mask them one by one. Insert a new column, give each person a role label, then delete the original column completely. In Excel or Google Sheets, with the employee in row 2:

="Employee "&ROW()-1

Copy the new column, paste it back as values, and delete the column that held the National Insurance numbers. Hiding the column is not enough: hidden columns travel with the file, as our guide to redacting an Excel spreadsheet shows.

In a PDF, a black box is not redaction

A black rectangle drawn over a number in a PDF is a shape on top of the text. The number usually stays underneath, and it comes back with a copy and paste. Use a real redaction tool, then check the result by searching for the number. Our guide on how to redact a PDF walks through it, and the same applies to a Word document, where comments and tracked changes can keep a deleted number alive.

Redacted or pseudonymised: what you have done when you finish

Once the number is out, it is tempting to call the file anonymised. The ICO uses that word for something stricter. In its innovation advice it says that removing direct identifiers such as a name or an identification number is insufficient to ensure effective anonymisation, and that information an organisation can still link back to a person is pseudonymous, not anonymous.

Article 4(5) of the UK GDPR defines pseudonymisation as processing personal data so that it can no longer be attributed to a specific person without additional information, provided that information is kept separately and protected. If you keep a note of which placeholder stands for which person, that note is the additional information.

Why pseudonymisation is the useful one

This is good news rather than a catch. Pseudonymisation is exactly what you want when you need the AI’s answer back with the real names in it. The placeholders let the model keep track of who is who, and the key lets you restore the details once the answer comes back to your own computer.

The file stays personal data while you hold the key, and it stays your responsibility, which is where it was before you started. Our guide to pseudonymisation under the UK GDPR explains how the ICO expects the key and the text to be kept apart.

What sits next to the number on a payslip

A National Insurance number almost never travels alone. On a payslip or a P60 it sits beside a name, a date of birth, an address and often an employer, and once the number has gone those are what still point at one person.

on the page	what to do before the AI sees it
Name, in every form it appears	replace with a role: the employee
Date of birth	an age band, if the question needs one
Home address and postcode	remove; a region is enough
Unique Taxpayer Reference	replace, like the NI number
Employer and job title	generalise if the workforce is small
Bank details	remove
Tax code and amounts	keep, if the question is about them

Our own checklist, built on the ICO’s guidance on indirect identification.

The last row matters as much as the others. Taking out everything that identifies someone is only half the job; the other half is leaving in what the question needs, or the answer comes back generic and someone pastes the original to get a better one.

The UTR, its sibling from HMRC

The Unique Taxpayer Reference is the other HMRC number that shows up in an accountant’s files. GOV.UK describes it as a 10 digit number, sometimes just called the tax reference, issued when someone registers for Self Assessment or sets up a limited company. Treat it exactly like the NI number: it identifies one taxpayer and adds nothing to a question. Written after its label, as UTR: 12345 67890, Nonimo replaces it with [UTR_1].

Pension and benefit letters

Letters are harder than payslips because the number is not in a box. It sits in a reference line under the address, or in the first sentence: your National Insurance number is quoted above, please use it in all correspondence. A letter also tends to carry the amounts, the dates and the name of the scheme or the benefit, which together can be as telling as the number.

For a question about a letter, the useful part is usually what it asks the reader to do and by when. Copy that part into the prompt, describe the rest in a sentence, and leave the header behind. The same goes for a photo of the letter, which a chatbot keeps like any other upload.

The full list of what to take out of a UK document, from the NHS number to the driving licence number, is in our guide to what personal data to remove before AI. The ICO’s point about combinations applies here too: an age, an occupation and a place of residence together can single someone out without any number at all.

Who handles National Insurance numbers every day

Some offices see a handful of NI numbers a year. Others see hundreds every month, and those are the offices where a quick paste into a chatbot is most likely to carry one along.

Payroll bureaux and accountants meet the number in every payroll run, every P60 and most Self Assessment files. HR teams hold it from the first day of employment, on starter details and pension enrolment. Solicitors find it in employment, family and personal injury files, usually inside a payslip or a pension letter that the client forwarded. Mortgage and pension advisers see it on applications and transfer paperwork.

Councils see it on Housing Benefit claims, since GOV.UK lists the local council among the bodies that may need the number when you claim. Councils have their own guide to AI in local government, and the Nonimo page for councils shows the same redaction on a benefit file.

For a solicitor, confidentiality comes first

Paragraph 6.3 of the SRA Code of Conduct for Solicitors says you keep the affairs of current and former clients confidential unless disclosure is required or permitted by law or the client consents. A National Insurance number in an employment or divorce file is part of those affairs.

6.3

The SRA published a warning notice on the misuse of AI on 17 August 2026. Among the issues it lists are instances of confidential client information being entered into public AI tools, risking potential breaches of confidentiality and wider data protection requirements. A firm that replaces the number, and the client details around it, before anything leaves the desk is already acting on that concern.

Our page for solicitors shows it on a client file, and the guide to an AI clause in the engagement letter covers telling clients about it.

For a payroll bureau, it is your client’s data

A bureau running payroll for other businesses usually processes the employees’ details on its clients’ behalf. That makes the payroll export your client’s data in your hands, and the most natural thing to do with it is keep it to the purpose your client gave you. Replacing the NI number before a query goes to an AI tool keeps that promise without slowing the query down.

What the Nonimo app does with a National Insurance number

Drop a document into the Nonimo app, or select some text, and it gives you back the same text with each personal detail replaced by a placeholder. It pseudonymises: the key stays encrypted on your own computer, and when the AI replies, the app puts the real details back. Here is a payroll query, invented from start to finish, before and after:

The query as written

Employee: Mrs Jane Bloggs
NI number: QQ 12 34 56 C
Date of birth: 30/02/1978
Email: jane.bloggs@example.com
Tax code: 1257L
Mrs Jane Bloggs asks why her September pay is lower than her August pay.

What the AI receives

Employee: [PERSON_1]
NI number: [REFERENCE_1]
Date of birth: [BIRTH_DATE_1]
Email: [EMAIL_1]
Tax code: 1257L
[PERSON_1] asks why her September pay is lower than her August pay.

The date of birth, 30/02/1978, a day that never existed, became [BIRTH_DATE_1]. The tax code came back unchanged, because a code describes an allowance rather than a person. The name came back as the same [PERSON_1] in both places, so the model still knows the employee in the last line is the one in the first.

Labelled, spaced or unspaced

A real number is recognised after the labels people actually use, National Insurance number and NI number, spaced or unspaced, and inside a sentence that says her NI number is, and it comes back as [NINO_1]. The government’s specimen after one of those labels, and numbers HMRC says are not NI numbers, such as those starting PZ, are still replaced, as a general reference.

The button at the top of this page leads to our guide with a box for pasted text, which redacts that text and discards it as soon as you have the result. For client files, the app does the same work on your own computer, so the document never leaves your desk. What it stores and what it sends is set out on our security page.

Before you paste: the ten second check

Whatever tool you use, the last look before you press enter is the one that counts. Search the text for two letters followed by six digits. Search for the word National. Look at the first five lines and the last five, because that is where headers, signatures and reference lines live, and check that the file you are about to attach is the copy you cleaned rather than the original.

If your team uses Copilot inside Microsoft 365 rather than a chatbot in a browser, the same check applies to whatever you ask it to read, and our guide on whether Copilot is safe for confidential information explains what it can see. And before a file goes anywhere, its properties can carry names the page does not show, which our guide to removing metadata from PDF and Word files covers.

---

Nonimo is the software that does this on your own computer: it replaces names, National Insurance numbers and client details before the text reaches ChatGPT or Copilot, and puts them back when the answer comes home.

Sources

Checked 1 October 2026.

• UK GDPR, Article 4, legislation.gov.uk: personal data and the identifiers in Article 4(1), including an identification number; the definition of pseudonymisation in Article 4(5).
• UK GDPR, Article 5, legislation.gov.uk: data minimisation in Article 5(1)(c), adequate, relevant and limited to what is necessary.
• UK GDPR, Article 9, legislation.gov.uk: the special categories of personal data.
• UK GDPR, Article 87, legislation.gov.uk: processing of the national identification number, omitted from 31 December 2020 by S.I. 2019/419.
• Data Protection Act 2018, section 3, legislation.gov.uk: personal data and an identifiable living individual, identified by reference to an identifier such as a name or an identification number.
• ICO, Can we identify an individual indirectly?: national insurance number listed as information that allows an individual to be singled out; combinations of age, occupation and place of residence.
• ICO, Innovation advice: previously asked questions: removing direct identifiers such as a name or an identification number is insufficient to ensure effective anonymisation; linkable information is pseudonymous, not anonymous.
• ICO, Pseudonymisation: the Article 4(5) definition as the ICO explains it.
• ICO, reprimand to ACRO Criminal Records Office, 7 August 2026: national insurance numbers among the identification numbers staged for extraction; 10,920 people affected at most; 84,048 notified; 35 complaints evidenced, including concern about identity theft and financial loss.
• HMRC, National Insurance Manual, NIM39110: two letters, six numbers and a suffix of A, B, C or D; the example QQ 12 34 56 A; letters and prefixes not used; administrative prefixes OO, FY, NC, PY and PZ, and the reference PP999999P.
• GOV.UK, Your National Insurance number: the example QQ123456B; the number stays the same for life; do not share it with anyone who does not need it, to prevent identity fraud; the list of organisations that may need it.
• GOV.UK, Find your National Insurance number: payslips, the P60 and letters about tax, pensions or benefits; HMRC cannot give the number over the phone or on webchat; the letter arrives shortly before the sixteenth birthday.
• GOV.UK Design System, National Insurance numbers: the specimen QQ 12 34 56 C used in the example on this page.
• GOV.UK, Find your UTR number: a 10 digit number, sometimes called the tax reference, issued on registering for Self Assessment or setting up a limited company.
• HMRC, Enquiry Manual, EM1516: HMRC’s own template letter prints 12345 67890 as the tax reference of XYZ Ltd, the sample UTR used on this page.
• SRA, Code of Conduct for Solicitors, RELs and RFLs: paragraph 6.3, keeping the affairs of current and former clients confidential.
• SRA, warning notice Misuse of AI, 17 August 2026: confidential client information entered into public AI tools.
• Nonimo, own measurement, 1 October 2026: the payroll query above and each of its lines, with the government’s specimen QQ 12 34 56 C; the labels National Insurance number and NI number with spaced and unspaced forms and the number inside a sentence, with the specimen and with a number in a valid shape that is not printed here; the administrative prefix PZ; and UTR: 12345 67890, run through the Nonimo engine.