A tax reference number is the unique identifier a tax authority assigns to a person or business so that every return, payment, and letter gets matched to the right account. In the United States, the Internal Revenue Service uses the umbrella term Taxpayer Identification Number, or TIN, for several nine-digit codes.1Internal Revenue Service. Taxpayer Identification Numbers (TIN) In the United Kingdom, HM Revenue and Customs issues a 10-digit Unique Taxpayer Reference, or UTR.2GOV.UK. Find Your UTR Number Which one you need, and how you get it, depends on who you are and where you file.
Which Number Applies to You
Federal law in the U.S. requires everyone who files a return, or is listed on someone else’s return, to include an identifying number.3Office of the Law Revision Counsel. 26 USC 6109 – Identifying Numbers The IRS recognizes several types.
The Social Security Number is the default identifier for U.S. citizens and permanent residents. The Social Security Administration issues it, but it doubles as your TIN on individual returns, W-2 forms, and most financial accounts.
The Employer Identification Number covers businesses, nonprofits, estates, and trusts. Banks generally require one before opening a business account, and any employer needs one to report payroll taxes.4Internal Revenue Service. Get an Employer Identification Number
The Individual Taxpayer Identification Number is for people who have a U.S. tax obligation but can’t get an SSN, including nonresident aliens with U.S. income and certain dependents or spouses of visa holders.5Internal Revenue Service. Individual Taxpayer Identification Number (ITIN) An ITIN has the same nine-digit format as an SSN but always begins with 9.
Two specialized numbers round out the system. An Adoption Taxpayer Identification Number lets adoptive parents claim a child while the adoption is pending and no SSN is yet available. A Preparer Tax Identification Number is required for anyone paid to prepare federal returns; if you hire a preparer, their PTIN should appear on your completed return.6Internal Revenue Service. Frequently Asked Questions – Do I Need a PTIN
In the U.K., the UTR works differently. The format is identical whether you’re a sole trader filing personal income or a limited company filing corporation tax; the distinction lives in HMRC’s records, not in the number itself. HMRC paperwork sometimes labels it simply as “tax reference” or “official use.”
How to Find a Number You Already Have
Tracking down an existing number is usually easier than applying for a new one.
U.S. Numbers
Your SSN appears on your Social Security card, prior-year Form 1040 returns, and any W-2 or 1099 forms from employers or financial institutions. If all of those are gone, the Social Security Administration can retrieve it once you produce identification.
For a lost EIN, the IRS suggests checking the confirmation notice from when you applied, contacting your bank, or reviewing previous business tax returns. If none of that works, call the IRS business line at 800-829-4933 and request a Letter 147C confirming the EIN, provided you’re authorized to receive it.7Internal Revenue Service. Employer Identification Number
ITIN holders can find their number on the original IRS assignment letter (CP 565) or on previously filed returns. The IRS will not release an ITIN over the phone without identity verification, so keep that assignment letter safe.
UK Unique Taxpayer Reference
You can find your UTR in your HMRC Personal Tax Account, through the HMRC app, or on previous correspondence such as payment reminders and notices to file.2GOV.UK. Find Your UTR Number The SA100 (the main Self Assessment return) and employment documents like the P60 and P45 also display it. If the paperwork is gone, logging into the Personal Tax Account online is the quickest route.
How to Get a New Number
Timelines vary sharply by identifier.
An EIN is the fastest. The IRS online application walks you through a short questionnaire about your entity type and issues the number in minutes once approved.4Internal Revenue Service. Get an Employer Identification Number Fax and mail applications take considerably longer.
An ITIN takes patience. You submit Form W-7 along with a federal tax return and supporting identity documents. In off-season months, the IRS estimates about seven weeks for processing. Between January 15 and April 30, or if you apply from outside the country, allow nine to eleven weeks.8Internal Revenue Service. How to Apply for an ITIN
A UTR comes from registering for Self Assessment through the HMRC website. HMRC sends the number by post, and delivery within the U.K. typically takes up to 15 days. If you live abroad, allow at least 21 days. You need the UTR before you can file a return or set up online access to your Self Assessment account.
When You’ll Be Asked for It
Your tax reference number comes up more often than at filing time. Any business or financial institution that pays you reportable income is required to collect your TIN, usually through a Form W-9.9Internal Revenue Service. About Form W-9, Request for Taxpayer Identification Number and Certification Banks, brokerages, freelance platforms, and rental property managers all ask for it so they can file accurate information returns.
When you make direct tax payments, including the correct reference number ensures the money is credited to the right account for the right tax year. And if you call the IRS or HMRC with a question, your identifier is the first thing the agent will ask for; it’s how they pull up your account and verify you’re authorized to discuss it.
What Happens If the Number Is Wrong or Missing
Failing to furnish a correct TIN carries a statutory penalty of $50 for each failure to comply with information reporting requirements involving your identifier, up to $100,000 per calendar year. These amounts are set by statute and are not adjusted for inflation.10Office of the Law Revision Counsel. 26 US Code 6723 – Failure to Comply With Other Information Reporting Requirements
The more immediate hit is backup withholding. When a payee doesn’t provide a valid TIN, or the IRS notifies the payer that the TIN is wrong, the payer must withhold 24% of the payment and send it to the IRS instead of paying you the full amount.11Internal Revenue Service. Publication 15 (2026), (Circular E), Employers Tax Guide The 24% rate was made permanent by legislation in 2025. You can claim the withheld amount back when you file your return, but in the meantime the money sits with the government.
Protecting Your Number From Identity Theft
Tax-related identity theft happens when someone uses your SSN or ITIN to file a fraudulent return and claim a refund in your name. The first sign is often that your legitimate e-filed return gets rejected because the IRS already has a return on file for that SSN. The IRS Taxpayer Protection Program may also send you a letter flagging a suspicious return and asking you to verify your identity before your real return is processed.12Internal Revenue Service. How IRS ID Theft Victim Assistance Works
If you’re a victim and need to file, complete Form 14039 (Identity Theft Affidavit), attach it to the back of a paper return, and mail it to the IRS. You can also submit Form 14039 online at irs.gov or by fax.13Internal Revenue Service. Identity Theft Affidavit (Form 14039) If the IRS sends you a Taxpayer Protection Program letter first, follow the instructions in that letter instead of filing Form 14039 separately.
Identity Protection PIN
The IRS Identity Protection PIN adds a second lock to your tax account. The IP PIN is a six-digit number generated fresh each year that you include on your return; without it, nobody can file using your SSN. Anyone with an SSN or ITIN can enroll through an IRS online account after verifying their identity.14Internal Revenue Service. Get an Identity Protection PIN
If you can’t create an online account, you may be eligible to apply by mail using Form 15227, provided your adjusted gross income on your last return was below $84,000 ($168,000 for married filing jointly).14Internal Revenue Service. Get an Identity Protection PIN Parents and legal guardians can also request an IP PIN for their dependents. It takes about ten minutes to set up and can save months of work if your information is ever compromised.