More than a million married couples across the UK are eligible to reduce their tax bill with the marriage allowance, according to HMRC.
The marriage allowance allows spouses, and couples in civil partnerships, to transfer £1,190 of their personal allowance to their partner, providing a tax break of up to £238 a year.
It can be claimed by couples where one partner pays the basic rate of income tax and the other is a non-taxpayer.
It can also be backdated to include any year since April 2015, to provide a total saving of up to £900.
Around 3 million couples have already claimed the relief, including more than 300,000 since March 2018.
HMRC has urged the remaining 1 million eligible couples to apply online.
Mel Stride, financial secretary to the Treasury, said:
"It's great news that so many couples are now benefitting from marriage allowance.
"This is a really important tax relief and reflects the social importance of marriages and civil partnerships.
"And I'd urge those that haven't yet managed to claim the money to do so right away - it's quick and easy to apply."
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