The National Insurance number (NI number) is the reference number by which HM Revenue & Customs (HMRC) identifies you in its records. In particular, it is the key to your National Insurance contributions record. Details of the National Insurance contributions (NICs) that you paid during each tax year are carefully recorded and used to calculate your state pension and additional state pension when you retire. The record is also used to decide whether you are entitled to certain state benefits, including Jobseekers Allowance and Incapacity Benefit.

In years up to 2004, employers were allowed to make up a "temporary" NI number if you did not provide a proper one. In most cases, it had the format TNDDMMYYG, where TN was short for "temporary number", DDMMYY was your date of birth, and G was your gender, M or F. The use of temporary numbers has been phased out by HMRC over a number of years and they may not now be used on any document sent to HMRC, particularly on returns that your employer submits at the end of the tax year.

At the end of each tax year, your employer sends details of the NICs that you have paid during the year to HMRC. The figures are entered on a P14 Summary and you are given a copy of those details on your P60 Certificate. If your employer is unable to enter your NI number on the P14, HMRC will be unable to create your contribution record for that tax year, unless your record can be traced using your name, date of birth and gender, details that your employer must by law include on the P14.

HMRC continually reminds employers of the need to obtain an NI number from every employee. There is a NI number tracing procedure that employers can use, but you should keep a careful record of your NI number and give it to your employer as soon as you start a new job.

If you have never been issued with an NI number and you wish to work in the UK, you are required by law to apply for one. See How do I obtain a National Insurance Number?

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