Nearly half of married couples are missing out on a tax allowance worth £230 a year, according to HM Revenue and Customs.
Since it was introduced in 2015, only 2.2 million couples have claimed the marriage tax allowance from 4.4 million eligible.
According to the Government the application process is now simpler.
Last year HMRC reported that only a quarter of eligible couples – married couples and those in a civil partnership – were claiming.
The latest figures were obtained as the result of a Freedom of Information request to HMRC by insurance company Royal London.
Steve Webb, director of policy at Royal London, said: “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.”
This is now the Marriage Allowance works:
Partners must either be married or in a civil partnership.
One partner must be working, and paying tax at the basic rate of 20%. If he or she is earning over £45,000 (£43,000 in Scotland) they are not eligible.
The other partner must be earning less than £11,500 in 2017-18, meaning they pay no tax.
If the above conditions are satisfied, the partner not paying tax can transfer 10% of his or her tax allowance to a partner, so saving £230 in this tax year.
Back-claims can be made for previous years.
HMRC has organised advertising campaigns to persuade people to apply for the allowance.
A spokesman for HMRC said: “Applications have increased year on year, and the application process is easy, and families can apply at a time which is convenient for them.”
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 and we have simplified the application process to make it easier for families to apply at a time which is convenient for them.”