National Insurance Number - Format

Format

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 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.