Only one in eight married couples have taken up David Cameron's much-hyped married tax allowance, thanks to lack of awareness and problems trying to apply.
Since April last year, married couples or civil partners have been allowed to transfer £1,060 of personal allowance from one to the other, as long as the person receiving it earns less than £10,600 a year.
Around 4.2 million couples are believed to qualify for the break, which can be worth up to £212 a year - but only half a million have signed up.
The Office for Budget Responsibility suggests that one reason may be lack of awareness, but also points to problems with HMRC's computer systems, especially in the first few months of the scheme.
Many users, particularly those without credit cards or driving licences, struggled to register or gave up trying altogether. And while it was later made possible to apply by phone, people reported long waits for help.
As a result, says the OBR, many people may have felt that it simply wasn't worth bothering, for the sake of an extra £3.85 a week.
However, Anthony Thomas, chairman of the Low Incomes Tax Reform Group (LITRG), says that it's a perk worth taking up.
"We urge you to make your claim, if you are entitled, and if doing so would save you tax. You will generally save tax by claiming if one of you has spare tax allowances that the other can make use of."
The application needs to be made by the person who doesn't pay tax, and can be made online at the GOV.UK website or by phoning 0300 200 3300. Alternatively, it's possible to apply by post: the LITRG has a template letter here.
You'll need both partners' National Insurance numbers and be able to pass identity checks that may include certain bank details or employment information. If all goes well, the process should only take a few minutes.