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The Conservatives are proposing to give four million married couples and civil partners an annual £150 tax break.Under their plan, the tax break would apply to basic rate taxpayers earning under £44,000 where one partner does not use their full personal allowance.
David Cameron has long called for the tax system to recognise marriage but has never before given details.
Labour said other families would lose out as a result and the Lib Dems called the estimated £550m plan "patronising".
The initiative comes at the end of the first week of election campaigning, which has been dominated by arguments over whose future tax and spending plans are more believable.
While Gordon Brown has accused the Tories of "spraying around" unfunded tax promises, the Tories have suggested Labour is keeping the truth about the impact of planned tax rises from the public.
The Tories would achieve the tax break by allowing some people who are married or in a civil partnership to transfer part of their tax free personal allowance to their spouse or partner.
That spouse would be able to transfer £750 of their personal tax-free allowance to their working partner, which the Tories say will represent a tax cut of about £150 a year for four million people.
'Symbolic'
The Tories say the independent Institute for Fiscal Studies (IFS) has judged that the policy would cost £550m annually and the tax break would be funded from the money raised on its planned levy on banks.
The Conservative commitment is to introduce the tax break during the next five-year parliament but they say they hope to introduce it in next year's Budget.
This is when they expect to introduce the bank levy which they hope will raise about £1bn a year by taxing risky borrowing. The rest of the money raised from the bank tax will go towards paying down the deficit.
Conservative sources said the tax break was designed to be a "symbol and message" that marriage was supported by the tax system.
But critics will say that it encourages one partner to stay at home and not work, and nor does it target help to children.
The IFS said 32% of all married couples were likely to qualify for the rebate but it was unsure people would choose to get married or stay together because of what was being offered.
"The incentives to marry - or not to divorce - provided by a policy whose maximum benefit is £150 a year must surely be weak relative to the other costs and benefits involved," it said.
It also suggested the plan would be "costly" to administer and there were simpler ways to help married couples on low and average incomes.
'Old-fashioned'
Labour says it supports marriage as a "really important institution" for bringing up children but that all forms of stable and loving relationships should be equally recognised.
"With the Conservatives you always need to look behind the headlines," Cabinet minister Douglas Alexander told ITV's News at Ten.
"What we see already this evening is what they're giving with one hand to married couples they're still taking away with the other hand from families with children right across the country."
The Lib Dems said the Tory proposals "belonged to the Edwardian age" and politicians should not be "dictating" to families how to live their lives.
"David Cameron clearly has no idea about modern life," the party's leader Nick Clegg said, adding that he would give more details of his own family policy on Saturday.
"Every family is different, and instead of creating rigid rules or special policies that help some families not others, we need a new approach from government."This article is from the BBC News website. ? British Broadcasting Corporation, The BBC is not responsible for the content of external internet sites.