National Insurance Numbers (NINOs): Format and Security: What a NINO looks like
A NINO is made up of 2 letters, 6 numbers and a final letter, which is always A, B, C, or D.
It looks something like this:
QQ 12 34 56 A
This is an example only and should not be used as an actual number.
https://www.gov.uk/hmrc-internal-manuals/national-insurance-manual/nim39110