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HMRC letter codes · National Insurance (CA)

CA5403Request your National Insurance number by post

Last reviewed 14 October 2026 · How Tax Letters checks its facts

In short

GOV.UK says to use form CA5403 to request a copy of your National Insurance number by post.

“Use form CA5403 to request a copy of your National Insurance number by post.”

GOV.UK: Get your National Insurance number by post · checked 14 October 2026

Information, not advice: this page reports what HMRC, GOV.UK and the bodies named publish about CA5403. Each fact has its source and the day it was checked beside it.

What it is

HMRC's National Insurance manual, on its page headed form CA5403, says a customer who still needs confirmation of their National Insurance number can fill in form CA5403 and send it to the address on the form.

“fill in form CA5403 and send it to the address on the form”

HMRC National Insurance manual: NIM39205 - National Insurance numbers (NINOs): confirming a NINO: confirmation by individual - form CA5403 · checked 14 October 2026

What it asks

GOV.UK says that if you cannot find your National Insurance number on any of your documents or by using the online service, you can fill in the CA5403 form online, print it, sign the declaration, include any documents to confirm your identity and post it to HMRC.

“If you cannot find your National Insurance number on any of your documents or by using the online service, you can: … Fill in the form online. … Print it. … Sign the declaration. … Include any documents to confirm your identity. … Post it to HMRC.”

GOV.UK: Get your National Insurance number by post · checked 14 October 2026

How to send it or apply

HMRC’s National Insurance manual, on its page headed form CA5403, lists phoning the National Insurance numbers helpline as one way to get confirmation of your National Insurance number. It says HMRC will post the number rather than tell it over the phone, and that it will arrive within 10 working days.

“phone the National Insurance numbers helpline and answer some questions … won’t tell customers their National Insurance number over the phone. They’ll post it to the customer and it will arrive within 10 working days.”

HMRC National Insurance manual: NIM39205 - National Insurance numbers (NINOs): confirming a NINO: confirmation by individual - form CA5403 · checked 14 October 2026

Free help and HMRC contacts

GOV.UK's CA5403 page says your National Insurance number will be on tax-related documents like your payslip or P60, and that you can also find your National Insurance number and get a copy of it online.

“Your National Insurance number will be on tax-related documents like your Payslip or P60. You can also find your National Insurance number and get a copy of it online.”

GOV.UK: Get your National Insurance number by post · checked 14 October 2026

Free help first

These routes are described by GOV.UK, TaxAid and Citizens Advice themselves.

  • HMRC's extra supportHMRC offers extra support if your health condition or personal circumstances make it difficult to contact HMRC, for example a mental health condition, financial hardship or being treated in hospital.
  • Someone to deal with HMRC for youYou can authorise a friend, relative or voluntary organisation to talk to HMRC for you and help you fill in forms.
  • TaxAidTaxAid helps people with tax problems who cannot pay for advice. Tax Help for Older People merged with TaxAid in July 2025.
  • Citizens AdviceCitizens Advice gives confidential advice online, over the phone and in person, for free.