Martin Lewis has issued an urgent warning to married people who risk missing out up to £1,250.
The MoneySavingExpert.com founder explained how around two million eligible couples are not claiming marriage tax allowance.
Marriage tax allowance allows you to transfer £1,260 of your personal allowance to your partner to cut their yearly tax bill.
Your personal allowance is the amount you can earn tax-free each tax year.
For the current 2022/23 tax year, the tax break you can get is worth £252 - but it is possible to claim back for all four previous tax years.
If you claim for this tax year and backdate the maximum four years, you'll get up to £1,242.
You need to be married or in a civil partnership to claim marriage tax allowance, and both of you must also have been born on or after April 6, 1935.
One of you also needs to be a non-taxpayer while the other person needs to be paying the basic 20% rate of tax.
This usually means one of you doesn’t pay tax or earns less than £12,570, while the other person would be making between £12,571 and £50,270.
Martin broke down who is eligible for marriage tax allowance in a message on Twitter this week - and urged anyone who is able to claim, to do so now.
The end of the current tax year is tomorrow (April 5) and after this date, you'll only be able to backdate your claim to the 2019/2020 tax year.
At the moment, you can go as far back as the 2018/2019 tax year.
He said: "URGENT pls share. Are you married or in civil partnership if so - Is one of you a basic 20% taxpayer?
“The other a non-taxpayer (earn under c£12,570/yr) If so ur [sic] likely due up to £1,300 tax back, but if you don't sort by Wed 5 April you risk losing £100s."
Martin added: "Many asking me 'We're pensioners can we do this', 'I'm a volunteer can I do this', 'I'm on maternity can I do this', 'I'm self employed can I do this', YES!
“It's simple, if you're under 85, and in a tax-year don't pay tax, then you can give this to your 20% tax rate paying spouse."
You can apply for marriage tax allowance through the HMRC website or by calling 0300 200 3300.
The person who earns the least should make the claim.
Sadly, you cannot claim marriage tax allowance if you’re living together but not married or in a civil partnership.
