David Cameron gave the pledge yesterday as a curtain-raiser to the Conservative annual conference that opens today in Manchester.
The Prime Minister said the tax breaks would apply to both gay and straight couples, whether married or in civil partnerships, to recognise their "commitment and responsibility".
About four million couples are expected to benefit from the change, which comes into effect on the eve of the next General Election.
Someone not using their full personal tax-free allowance, because they do not work or earn less than £10,000, could transfer £1000 of their allowance to their spouse or partner. The figure is higher than the £750 promised in the Conservatives' 2010 manifesto.
However, the tax break, worth about £200 a year, will not be available to couples including a higher-rate taxpayer, which in 2015 means anyone earning more than £42,285.
The plan, to be confirmed in the autumn Budget statement, restores a recognition of marriage in the tax system that was lost in 1999, when a similar allowance was phased out.
It is aimed in particular at women working part-time and stay-at-home parents. It also makes good on a promise Cameron gave when he ran for the Tory leadership in 2005.
Writing in yesterday's Daily Mail, the Prime Minister said: "There is something special about marriage: it is a declaration of commitment, responsibility and stability that helps to bind families. The values of marriage are give and take, support and sacrifice - values that we need more of in this country."
Labour's Rachel Reeves, shadow chief secretary to the Treasury, said the tax break would not help two-thirds of married couples, or the separated, divorced or widowed in need. "He's so out of touch he thinks people will get married for £3.85 a week," she said.
"Even for the minority who might benefit, it will be far outweighed by what David Cameron's government has already taken away in higher VAT and cuts to child benefit and tax credits."
l Scots Tory leader Ruth Davidson will set out more plans to give parents greater choice over which school their children attend in a speech at her party's UK conference tomorrow. Davidson is expected to develop the idea of "opportunity vouchers", which would let parents move children from failing state schools into better-performing ones.