HMRC letter codes · National Insurance (CA)
CA5610Refund of self-employed National Insurance, profits-based
Last reviewed 14 October 2026 · How Tax Letters checks its facts
In short
GOV.UK lists the CA5610 as the form to apply for a refund of Class 4 National Insurance contributions.
“Apply for refund of Class 4 National Insurance contributions (CA5610)”
GOV.UK: Apply for refund of Class 4 National Insurance contributions (CA5610) · checked 14 October 2026
Information, not advice: this page reports what HMRC, GOV.UK and the bodies named publish about CA5610. Each fact has its source and the day it was checked beside it.
What it asks
GOV.UK's CA5610 page says you need to fill in the postal form fully before you can print and post it, and that you cannot save a partly filled in form.
“You’ll need to fill in the postal form fully before you can print and post it. You cannot save a partly filled in form”
GOV.UK: Apply for refund of Class 4 National Insurance contributions (CA5610) · checked 14 October 2026
How to send it or apply
GOV.UK's CA5610 page says that to apply for a refund of Class 4 National Insurance contributions you can use the online service or fill in the postal form on-screen, print and post it to HMRC.
“To apply for a refund of Class 4 National Insurance contributions you can: … use the online service … fill in the postal form on-screen, print and post it to HMRC”
GOV.UK: Apply for refund of Class 4 National Insurance contributions (CA5610) · checked 14 October 2026
GOV.UK's CA5610 page says that to apply online you need to sign in, and that you can create sign in details when you apply if you do not already have them.
“To apply online, you’ll need to sign in. If you do not already have sign in details, you’ll be able to create them when you apply.”
GOV.UK: Apply for refund of Class 4 National Insurance contributions (CA5610) · checked 14 October 2026
GOV.UK's CA5610 page says that if you use the online form you will get a reference number you can use to track the progress of your form.
“If you use the online form, you’ll get a reference number that you can use to track the progress of your form.”
GOV.UK: Apply for refund of Class 4 National Insurance contributions (CA5610) · checked 14 October 2026
Free help and HMRC contacts
If you have a Class 4 National Insurance query, such as one about CA5610, GOV.UK’s National Insurance enquiries page says to call HMRC on 0300 200 3310. Lines are open Monday to Friday, 8am to 6pm, and the page links to “Find out about call charges”.
“Call HMRC for help with Class 4 National Insurance queries. … Telephone: 0300 200 3310 … Opening times: Monday to Friday, 8am to 6pm. … Find out about call charges”
GOV.UK: National Insurance: enquiries · 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.
- GOV.UK: Apply for refund of Class 4 National Insurance contributions (CA5610) (checked 14 Oct 2026)
- GOV.UK: National Insurance: enquiries (checked 14 Oct 2026)
- GOV.UK: Get help from HMRC if you need extra support (checked 7 Oct 2026)
- GOV.UK: Appoint someone to deal with HMRC on your behalf (checked 7 Oct 2026)
- TaxAid: About us (checked 8 Oct 2026)
- Citizens Advice: About us (checked 7 Oct 2026)