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.
Mr 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.
"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."
His criticism was joined by charities and Labour. Shadow Treasury minister Catherine McKinnell said: "At a time when the Government's failed economic policies mean living standards are falling, we should be helping all families and not just some. Millions of people who are separated, widowed or divorced, as well as married couples where both partners work and use all their personal allowance, won't get any help from this out-of-touch policy."
Fiona Weir, chief executive of single parent charity Gingerbread, said: "It is astonishing, just days after the Chancellor set out billions of pounds of cuts, that the Government intends to push ahead with a tax break that favours one family type over another and costs hundreds of millions of pounds."
The push for the marriage tax pledge to be implemented is being led by Tory former minister Tim Loughton, who has tabled an amendment to the Finance Bill which is likely to spark a Conservative rebellion if it is pressed to a vote on Tuesday. He said he was "delighted" that Mr Cameron appeared ready to bring forward plans but was still awaiting fuller details from Downing Street about its plans before deciding whether to do so.