The National Insurance number is a number used in the United Kingdom in the administration of the National Insurance or social security system. It is also used for some purposes in the UK tax system. The number is described by the United Kingdom government as a "personal account number". Because it is the only number allocated to almost every adult resident of the country, it is used for some limited purposes to check identity (the NHS number can fulfil a similar role in England & Wales but Scotland and Northern Ireland don't use them).
The number is sometimes referred to as a NI No or NINO.
Allocation of number 
People born and resident in the UK are assigned an NI number shortly before their 16th birthday. However, allocation of this number might occur a long time before this occasion (the date can usually be established from the prefix letters used), and siblings may have consecutive numbers - this is dependent on the payment of Child Benefit.
Persons from abroad who wish to work in the UK, or those to whom a number was not initially allocated as children, may apply for a number through the Department for Work and Pensions (DWP). The prefixes used are typically different from those used in the normal run.
The format of the number is two prefix letters, six digits, and one suffix letter. The example used is typically AB123456C. Often, the number is printed with spaces to pair off the digits, like this: AB 12 34 56 C.
Neither of the first two letters can be D, F, I, Q, U or V. The second letter also cannot be O. The prefixes BG, GB, NK, KN, TN, NT and ZZ are not allocated. Validation lists of issued two-letter prefixes are published from time to time.
After the two prefix letters, the six digits are issued sequentially from 00 00 00 to 99 99 99. The last two digits determine the day of the week on which various social security benefits are payable and when unemployed claimants need to attend their Jobcentre to sign on (renew their claims): 00 to 19 for Monday, 20 to 39 for Tuesday, 40 to 59 for Wednesday, 60 to 79 for Thursday and 80 to 99 for Friday.
The suffix letter is either A, B, C or D. The NI number is unique without the suffix letter, so, for example, if AB 12 34 56 C exists, then there will be no other numbers beginning with AB 12 34 56. In official electronic submissions, the final letter may be represented by a space if not known.
Until 1975, the suffixes A, B, C and D at the end of the NI number signified the period of validity of the National Insurance cards originally used to collect National Insurance contributions (NICs). Cards were exchanged every twelve months and because of the very large numbers of cards issued the exchange was staggered. Suffix A cards ran from March of one year until March of the next when they were exchanged for a new one. Stagger B suffix cards ran from June until the following June, stagger C from September until the following September and stagger D from December until the following December. For example a B stagger card issued in 1955 might have run from the first Monday in June that year until the first Sunday in June the following year. This staggered system operated from 5 July 1948 until 1975, at which time the A stagger cards were extended to run an extra five weeks, until 5 April 1975, in line with the end of the tax year. The B, C and D stagger NI Cards had a shorter period of validity in their final year, and ran from June, September and December respectively in 1974 until 6 April 1975. From 6 April 1975 onwards, a computerised National Insurance Recording System (NIRS) was used to allocate all NICs by tax years.
In Great Britain, expired NI cards were sorted into one hundred separate groups corresponding to the final two numbers of the NI number and were posted to the individual insured person's NI account (the RF1) by the corresponding one hundred ledger sections at the Records Branch of the Central Office of the Ministry of Pensions and National Insurance and its later incarnations - Ministry of Social Security, Department of Health and Social Security, Department of Social Security, Contributions Agency, etc. These 100 sections dealt not only with the recording of NI contributions but with requests for information about qualifying contributions necessary to pay sickness, unemployment, widows and other benefits and also with any correspondence arising from those NI accounts and NI cards. Within each of the 100 sections, NI numbers were allocated among 16 splits with one clerk administering each split. To trace unknown NI numbers, a general index contained millions of small RF2 index slips, filed in order of surname and listing the name(s), date of birth and NI number for every person within the National Insurance scheme.
The Northern Ireland National Insurance scheme is funded and administered separately from the Great Britain scheme but operates identically so that, in practice, the same rules apply throughout the UK.
Temporary numbers 
In the past, employers sometimes allocated their employees a temporary insurance number, which followed the format "TN dd mm yy x", where 'TN' stands for temporary number and is static and x is usually M for male or F for female and the numbers in the midsection are the employee's date of birth. In the case of a woman born on 31 December 1958, for example, the temporary NI number would most likely be TN 31 12 58 F. Temporary NI numbers cannot be used to trace back any NI credits or personal details. The National Insurance number must be obtained - the temporary code must not now be used.
Another type of temporary NI number is the Revenue-issued "temporary reference" used when HMRC is unable to trace a taxpayer's original NI number. It follows the format 63T12345.
Administrative numbers 
Reference numbers similar in format to NI numbers are sometimes allocated for tax or benefit purposes with special prefix letters. Special prefixes used in the past include the letters OO (for tax credit claims), CR (for investigations), FY (formerly for Attendance Allowance claims, named after the Fylde social security office where claims were processed), MW (used from 1980 to 1987 for migrant workers), NC (formerly for stakeholder pensions), PP (for use by pension schemes as PP999999P), and PY or PZ (both used for tax-only accounts created prior to 2003).
From 1984 until 2011, when a person was allocated an NI number he or she also received a plastic 'numbercard' of similar proportions to a credit card with the number raised on the front. Prior to 1984, a manila notification card was issued instead. The card is only used as a reminder of the number and the card itself is not needed to start work, and is not considered a valid identity card. Numbercards were phased out from September 2010, and the issue of numbercards ceased completely in October 2011. NI numbers are now notified by letter.
Crown dependencies 
National Insurance numbers issued in the Isle of Man hold the prefix MA. Similarly, those issued in Jersey start with JY, and those issued in Guernsey hold the prefix GY. Only Channel Island NINOs issued prior to 1975 are validated, and recognised for UK use by HMRC.
Use for tax purposes 
The National Insurance number is used as a reference number in the Pay As You Earn system, and also by the self-employed. It is also used in applications for Individual Savings Accounts (ISAs), to check that an individual has opened only one ISA in a tax year.
However, the NI number is not used universally as a tax identification number. Taxpayers who need to file a tax return are given a different number, a Unique Taxpayer Reference (UTR), which is used as a reference number in the self-assessment tax system.
Use for identification 
As the UK does not have a system of personal identity cards, and not everyone has a passport, the NI number is the only system which provides almost every adult in the country with a code number (the separate NHS administrations for each country also provide numbers that can perform a similar role within England, Scotland, Wales and Northern Ireland, but do not have an integrated database etc. Consequently, NI numbers are sometimes used for identification purposes in other contexts which have nothing to do with their original National Insurance purpose - such as forming part of evidence of right to work in the UK. The NI card, however, is not proof of identity.
- Directgov Retrieved 2012-06-07
- for example, by HM Revenue & Customs in National Insurance Manual NIM39100
- "HM Revenue & Customs: Applying for a National Insurance Number". Retrieved 26 February 2011.
- "Format and Security: What a NINO looks like". HM Revenue & Customs. Retrieved 29 September 2011.
- UK Government Data Standards Catalogue
- Message Implementation Guidelines for PAYE NINO Verification Request, Appendix B: Valid National Insurance Number Prefixes (page 46)
- List of valid National Insurance number prefixes, HM Revenue & Customs: Avoiding and correcting errors in your Employer Annual Return
- "Decision Makers' Guide: Chapter 08 - Payment of benefit/Deductions from benefit". Department for Work and Pensions. July 2011. Retrieved 29 September 2011.
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- HMRC: How to get a Self Assessment tax return
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- BBC News Northern Ireland 9 September 2002
- The Guardian 15 September 2010
- UK Government Data Standards Catalogue - National Insurance Number - The official UK government definition of the NI number format. Also includes links to the XML Schema data type definition in the CitizenIdentificationTypes schema published by the Office of the e-Envoy.
- Format and Security: What a NINO looks like HM Revenue & Customs official page
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