HMRC figures show just over three million couples have successfully applied for the marriage allowance but as many as nine million taxpayers could be eligible, according to analysis by Online Tax Rebates.

Tony Mills, director of Online Tax Rebates, said the low take up is mostly down to people not knowing the scheme exists.

“HMRC will not contact you if you’re eligible or advise on an individual basis if this tax relief is beneficial to you, so it’s up to you to work it out,” he said.

Here’s what you need to know:

What is the marriage tax allowance?

The marriage allowance lets you transfer part of your personal allowance to your partner if they earn more than you.

The personal allowance is the amount of income you don’t have to pay tax on so by transferring some your allowance to your partner, this means they are able to earn more income tax-free.

Who is eligible to receive the allowance?

You can transfer your personal allowance to your husband, wife or civil partner as long as you are a non-taxpayer, which typically means you earn less than £11,850 (or £12,500 from next April), and they are a basic rate taxpayer – so they earn less than £46,350 (or £50,000 from next April).

Co-habiting couples are not eligible.

Both partners must have been born on or after 6 April 1935.

How much can you get?

This tax year, the marriage allowance is worth £238 in your pay check. However, you are able to backdate your claim to include any tax year since 5 April 2015 that you were eligible. As the personal allowance has gradually increased since 2015, so too has the value of the marriage allowance. It means you could be entitled to £900, which you’d receive as a  lump sum.

Is the marriage allowance going up next year?

Yes. From April 2019, the personal allowance is going up to £12,500, which means you could pocket £250 a year.

How do you apply?

The easiest way to apply is online at the gov.uk website.

The non-taxpayer should complete the application. You can apply at any point during the tax year.

How does it work?

HMRC will either change you partner’s tax code to reflect their increased personal allowance (this can take up to two months) or they’ll get their extra allowance when they file their self-assessment form.

Do you need to apply every year?

No. Once you’ve applied, your allowance will transfer automatically every year unless you cancel it, one of you starts paying a higher rate of income tax or your marriage comes to an end.

Can you claim if you’re unemployed?

Yes, as long as your partner is in employment and a basic rate taxpayer.

What happens if you’ve transferred some of your personal allowance and your partner dies?

Their estate will be treated as having the increased personal allowance and your personal allowance will go back to the normal amount.

What happens if your partner transferred some of their personal allowance to you before they died?

Your personal allowance will remain at the higher level until the end of the tax year (5 April) and their estate will be treated as having the smaller amount.

Can you claim if your partner has died?

Yes, a new ruling means you can claim if your partner has died since April 2015 and you meet all the other eligibility criteria. It’s best to speak to an HMRC adviser over the phone in this situation.