Conservative plans to spend hundreds of millions of pounds on a tax break for married couples are "unfair", Nick Clegg has said.
In a fresh swipe at the policy - expected to be brought forward within months to head off a Tory revolt - the deputy prime minister said it would be paid for by people who do not "conform" to Conservative social norms.
And he suggested the cash should go instead on extending tax breaks on childcare for all couples.
At the time of the 2010 general election, David Cameron promised a tax break worth £150 to married couples by allowing the income tax allowances of non-working spouses to be transferred.
The prime minister indicated at the weekend that he would be announcing plans to implement it "quite shortly" in a move that appeared to have been sufficient to calm growing impatience for action among his MPs.
Clegg, who before the 2010 election ridiculed the proposal as "patronising drivel that belongs in the Edwardian age" secured a deal in the coalition agreement that Liberal Democrat MPs could abstain on any plan to recognise marriage in the tax system.
"I have never understood the virtue of a policy that basically says to people who are not married: you will pay more tax than people who are married or, more particularly, married according to the particular definition of marriage held by the Conservative Party," Clegg told journalists at a Westminster press conference.
"If you have got hundreds of millions of pounds to spend on tax breaks like that then I would much rather spend it on all working families to improve the tax breaks we are going to give them on childcare, for instance.
"We have offered, from 2015, tax-free childcare for working parents worth about £1,200 per child. I would like to see that expanded.
"Instead, for reasons that I have never quite understood, the Conservatives want to basically say to a widow...you are not going to benefit from a tax break even though you were married and you lost your husband.
"A woman who has been abandoned by her husband suddenly doesn't get the tax break even though she believes in marriage.
"This desire of the Conservative Party to hand-pick couples through the tax system who conform to their image of how you should conduct your life, I don't think it's fair.
"And I certainly don't think it's fair on all those other people who are going to have to pay higher taxes to fund this proposal."