Conservative marriage tax break plans condemned by Nick Clegg as "patronising drivel" will be brought forward soon, David Cameron has promised.
The concession comes as former minister Tim Loughton's amendment attempting to introduce the reform in the finance bill comes before the Commons this week.
Conservative plans to provide a £150 annual financial incentive for married couples were included in the 2010 coalition agreement as one of a limited number of areas where the Liberal Democrats were 'allowed to disagree' with their partners in government.
Clegg did that with enthusiasm this morning, during the first of his new monthly press conferences.
"I have never understood the virtue of a policy which basically says to people who aren't married : 'You will pay more tax than people who are married'," he said.
"If you've got hundreds of million of pounds to spend on a policy like that I'd rather offer a tax break for all working families.
"For reasons I've never understood, the Tories want to say to a widow: 'You will not benefit from a tax break'. A woman abandoned by her husband will not get a tax break."
With Labour also opposed, the issue could result in a bruising Commons debate for coalition unity later this year. Government ministers have preferred to come to a deal rather than openly disagree in the past.
Now Loughton's amendment has forced the prime minister to concede the switch, which chancellor George Osborne is thought to be lukewarm about, will be brought forward "shortly" - perhaps in this year's autumn statement.
"The point is that we are going to be putting in place the marriage tax proposal in law," David Cameron said.
"We will be announcing plans for that in this parliament, quite shortly in fact."
Loughton's amendment calls for a transferrable tax allowance. The ex-minister warns the current finance bill is the last chance for the measure if it is to come into force before the next general election.
"It is a surprise to hear that the prime minister has not pushed for this in the Queen's Speech given his repeated assurances he is committed to do it," Loughton said last week.
"It really is time for him to put our money where his mouth is. He really must end this discrimination against married couples in the tax system."
Under his proposals husbands or wives who earn less than £41,000 could add their spouses' personal allowance to their own if they do not pay any income tax themselves.
Such a move would result in an extra £150 for around four million couples, it is thought. But it would be up to the chancellor to decide the level of the transferable allowance, giving Osborne what Loughton calls "maximum flexibility".
"Last week the government announced £11.5 billion of cuts but this week they can find half a billion pounds for a marriage tax allowance to promote their fantasy 50s family, that's a married couple with a breadwinner and a homemaker," the Don't Judge My Family campaign's director Julianne Marriott said.
"It's out of step with modern families who come in all shapes and sizes and discriminates against families with single parents, widows and widowers, couples who both work and couples who chose not to marry."
The finance bill, which implements the bulk of the measures contained in the Budget, is at report stage in the Commons on Monday and Tuesday this week.