National Insurance Numbers (NINos): Format and Security: What is a NINo?
The NINo is an administrative number primarily for the use of HMRC and DWP. Many people have the same or similar identity details so having a unique NINo means that any Tax and National Insurance Contributions they pay or credits they are awarded can be correctly recorded on their NI account. It is used by DWP to check the NI record when that person makes a claim to contributory state benefit and to record information about benefit claims on the National Insurance account.
An individual is given one NINo - once it has been allocated they keep the same number forever.