An estimated 2.4 million couples in the UK are currently missing out on a tax break worth more than £1,000.
The marriage tax allowance allows a non-taxpaying spouse to shift 10% of their annual tax-free allowance to their basic-rate (20%) taxpaying partner.
Money Saving Expert says this is worth £252 this year, and you can claim back up to four tax years, as long as you were eligible during that time.
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This means backdating can get you as much as £970.
People in a civil partnership are also eligible for the allowance. Unfortunately, just living together doesn't count.
One of you needs to be a non-taxpayer, earning less than the £12,570 personal allowance between 6 April 2021 and 5 April 2022.
The other partner needs to be a basic 20% rate taxpayer.
This means you'd normally need to earn less than £50,270. Higher or additional-rate taxpayers aren't eligible for this allowance.
You both must also have been born on or after 6 April 1935.
If you are claiming the allowance for the current year, the higher earner will see it deducted in the form of paying less tax on their wages or salary.
If you are claiming for previous years, it will arrive via bank transfer or cheque.
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