If you decide to make a one-off payment of voluntary contributions, or you want to pay quarterly when you get a bill, you’ll need to contact HMRC’s National Insurance office on 0300 200 3500 and ask for an 18-digit reference number.
The lines are very busy and you may not always be able to get through to an advisor. If you can't get through, you can also pay by cheque with your National Insurance number as a reference.
If you’re not making National Insurance contributions and want to make voluntary contributions to ensure you don’t continually build up a gap in your National Insurance record, you can set up a Direct Debit to pay the money monthly.
For example, you might be employed but earning under £123 a week and not eligible for National Insurance credits. You don’t need to phone HMRC for a reference in this case.
You make Class 2 contributions if you’re self-employed, and most people make these contributions as part of their Self Assessment tax bill. However, if you don’t pay through Self Assessment, you can use the ‘Pay now’ link on the government’s Pay voluntary Class 2 National Insurance page. You’ll need:
To pay Class 3 contributions if you need to fill gaps in your record, use the ‘Pay now’ link on the government’s Pay voluntary Class 3 National Insurance page.
You can usually pay voluntary contributions to fill gaps for the past six years. Check the GOV.UK page for payment deadlines for voluntary contributions