National Insurance number
What it really means
Your personal reference number for the tax and benefits system, made of letters and digits like AB123456C. You need one to work legally and be taxed correctly. You keep the same number for life; apply on GOV.UK if you do not have one.
Context, tips & common mistakes
You can legally start work before your number arrives as long as you can prove your right to work — tell the employer it is 'applied for' and give it to payroll when it comes, so do not turn down a job offer just because the letter is still in the post. Apply only through the official GOV.UK service, which is free: search results often surface lookalike sites charging £30 or more for the same application, a classic newcomer trap. Some visa holders find the number already printed on the back of their biometric residence permit, in which case no application is needed at all. Treat the number like a password — legitimate organisations that need it are essentially your employer, HMRC and certain government services; cold callers claiming your 'NI number has been compromised' and demanding payment are a well-known phone scam. If you ever lose track of it, it appears on payslips, P60s and your personal tax account.