Backbench Tories have urged the Prime Minister to keep his promise to give married couples a tax break.
But the Government says now is not the right time for the measure because it would be too costly in the current economic climate.
The MPs want husbands and wives to be able to share income tax allowances – a move that could benefit couples by up to nearly £1,500.
The issue was the topic of a debate in the House of Commons last night led by Edward Leigh MP.
Mr Leigh said the Government was being too slow in keeping its promise to support marriage in the tax system.
He told MPs: “The prime minister and other senior Conservatives repeatedly expressed their commitment to recognise marriage in the tax system during the last parliament.
“There were some very strong statements, particularly by the prime minister.”
He said tax breaks for married couples were “the key policy response to the challenge of social breakdown, the ‘broken Britain’ phenomenon”, and a manifesto pledge on which every Conservative MP stood.
He urged the Government to “fulfil the pledge they made solemnly in the manifesto, that they put in the coalition agreement and which we are still waiting for”.
Treasury Minister, David Gauke MP, responded for the Government. He said the Government supports the principle of tax breaks for married couples.
But he said that “now is not the appropriate time to bring forward such a measure” because it “would entail significant and immediate costs to the Exchequer.”
He added: “it is not practical to implement it at this time, and such changes need to be made within the boundaries of improved fiscal stability.”
The Labour Party hit out at the idea of a tax break for married couples.
Labour Treasury spokesman David Hanson said: “It is astonishing that at a time when millions of families and pensioners are being hit hard by deep spending cuts and tax rises, the first priority of David Cameron’s restless Tory backbenchers is unfair tax cuts only for a few.
“And the proposed multi-billion pound marriage tax break would penalise those who are separated, widowed or divorced – many of whom are already being hit hard by cuts to tax credits and childcare support.”
Mr Leigh pushed for a vote on the amendment tabled by Fiona Bruce MP that would introduce a tax break for married couples.
The amendment secured the support of 23 MPs, but opposition from the coalition Government and Labour meant 473 MPs voted against it.