Schools Secretary Ed Balls accused David Cameron of engaging in "social engineering" and Liberal Democrat leader Nick Clegg called the measure an unfair "bribe".
But Mr Cameron renewed his commitment to the policy, saying he wanted to send out a message that "If you take responsibility, you'll be rewarded".
The Conservative leader is under mounting pressure to explain what form the financial help will take after admitting the country cannot afford a blanket shared allowance for all spouses.
And opponents are keen to exploit Mr Cameron's recent gaffe when he appeared to signal the policy was no longer a firm commitment before insisting it would be implemented in a Tory first term.
As family policy took centre stage in pre general-election skirmishes, Mr Cameron said supporting marriage was an important part of his plans to tackle Britain's "broken society".
"A stable home is the best start a child can get. That's why we'll back commitment by recognising marriage in the tax system - and we'll also end the couple penalty in the tax credits system which, unbelievably, encourages parents to live apart," he wrote in the Mail on Sunday.
Mr Balls said he agreed with the Tories that marriage was "the best way to bring up children" but argued the tax break would punish many good parents and stigmatise their children.
He will unveil measures this week to help families "of all shapes and sizes" - including a "New Dad's Guide" aimed at encouraging new fathers to get more involved with childbirth and new-borns.
"The idea of trying to socially engineer family life through a tax policy which is designed to say that some types of families are first class and other types of families are second class and should be financially disadvantaged is hugely expensive and unfair," he told the Sunday Telegraph.
"I don't think children should be told they are second-class kids because of things which have happened through no fault of their own or unavoidable reasons."
Mr Clegg said it was "plain wrong...to say that we, the country, should spend billions of pounds providing a tax bribe for people simply to hold up a marriage certificate."
"What does it mean for the poor woman who has been left by some philandering husband who goes on to another marriage and gets the tax break and she doesn't," he told the BBC.
The best way to get fathers involved at an early stage was to allow them to share in 19 months of parental leave with the child's mother, he said, rather than traditional maternity leave.
David Willetts, the shadow cabinet minister responsible for family policy, said that "one of the most powerful arguments" for restricting the benefit to married couples was that it would be less "intrusive" than the taxman trying to judge the eligibility of other family units.
He confirmed that no tax break would be included in the Tories' initial budget on taking office as "we simply don't have the resources".
But former Tory leader Iain Duncan Smith renewed his calls for at least a minimum measure to be introduced "as a matter of urgency".
The MP, whose Centre for Social Justice (CSJ) thinktank developed the proposals for a transferable income tax allowance, believes a Tory administration could begin a staged introduction as soon as its second year in office.
Giving it to those married couples with children aged under three could be done for £600 million, he believes, as opposed to the £3.2 billion needed to extend it to all married couples.
The call is contained in a report drawing together CSJ proposals to cure what it has dubbed "breakdown Britain", many of which are expected to be adopted by Mr Cameron."
Shadow chief secretary to the Treasury Philip Hammond said: "Our approach is evidence-based: studies prove beyond doubt that children do better when their parents are together - and parents of young children are nearly four times more likely to stay together if they are married.
"Britain is alone among the major developed economies in not recognising marriage in the tax system. So the next Conservative government will introduce a specific recognition of marriage into our tax system to improve children's life chances, encourage social mobility and reduce the demands on the state from social breakdown, as David Cameron pledged to do as long ago as 2005".
Mr Balls denied the handbook for fathers, to be unveiled as part of a Families Green Paper on Wednesday, would be resented as a "nanny state" measure.
Officials said the document, 600,000 of which are to be distributed, would contain information on issues such as: "explaining why Dads matter"; "communication with baby and keeping them safe"; "providing financially"; and "keeping a good relationship with Mum".
"The Royal College of Midwives say...that we have not done enough to support dads and their role in childbirth in the months and weeks beforehand," he said.
"If dads aren't engaged and involved that can be the time they walk away. All the evidence is that if fathers are properly engaged and involved at that time, then they stay, they are supportive to their children, they do all the things which lead to better outcomes."
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