Nearly half the couples eligible to claim marriage tax allowance are still failing to do so, according to HM Revenue and Customs
The tax allowance - worth up to £230 a year - can be claimed by married couple and those in civil partnerships, but certain criteria has to be met.
When the 'marriage allowance' was introduced in 2015/16, it was estimated that 4.2 million couples stood to gain.
However figures, obtained in a Freedom of Information request to HMRC by insurance company Royal London, show that only 2.2 million couples have so far claimed - suggesting around two million more could still benefit.
The allowance aims to help married couples and members of civil partnerships where one partner pays standard rate income tax. If they are earning more than £45,000 then they are not eligible.
The other partner must be earning less than £11,500, meaning they pay no tax.
The partner paying no tax can then transfer 10 per cent of his or her tax allowance to the other person, so saving £230 in this tax year.
Steve Webb, a former pensions minister who is now director of policy at Royal London, said: "The take-up of the new allowance is shockingly low. Even in its third year of operation, around two million couples who could benefit from the marriage allowance are not doing so.
"When family finances are so tight, I would encourage every married couple to check whether they might be eligible, including for the last two years, as they could qualify for a useful lump sum as well as a reduction in their ongoing tax bill."
A Government spokesman said: "2.2 million couples across the UK are keeping more of the money they earn thanks to the marriage allowance.
"Applications have increased year-on-year, the application process is easy and families can apply when convenient for them."
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