Nearly half the couples eligible to claim marriage tax allowance are still failing to do so, according to HM Revenue and Customs (HMRC).
The tax allowance - worth £230 a year - can be claimed by married couples or those in a civil partnership if they meet certain conditions.
However, since it was introduced in 2015, only 2.2 million couples have claimed it, from 4.4 million eligible.
The government said it had now simplified the application process. Last year, HMRC reported that only a quarter of eligible couples were claiming.
The latest figures were obtained as the result of a Freedom of Information request to HMRC by insurance company Royal London.
How Marriage Allowance works
One partner needs to be earning at least £11,500 a year, 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.
HMRC has organised several advertising campaigns to persuade people to apply for Marriage Allowance, and is known to be frustrated by the lack of take-up.