But former Tory leader Iain Duncan Smith warned that any delay in implementing the policy would cause “an almighty uproad”.

And in what Labour claimed was a separate blow to the Tory leader’s battle for the family, Treasury figures showed the Conservatives’ plan to save £400 million a year by scrapping means-tested tax credits for families on incomes over £50,000 will only bring in around £45m.

Liam Byrne, Chief Secretary to the Treasury, noted how Mr Cameron made marriage tax breaks the centrepiece of his leadership campaign four years ago but claimed the policy was now falling apart as the leader of the Opposition had no idea how to fund it.

Mr Byrne told The Herald: “It’s just another example of David Cameron being all talk and no action. The Tories’ tax plans are unfair, unfunded and are unravelling fast.”

Last night, a Conservative Party spokesman hit back, insisting: “We have made a commitment to recognise marriage in the tax system. Nothing has changed.”

On scrapping tax credits, he insisted that calculations from the Institute for Fiscal Studies, the respected think-tank, supported his party’s view on how much would be saved.

In a bid to bolster support among traditional Tories, Mr Cameron has emphasised how he wishes to support marriage in the tax system but critics say he is increasingly vague on how to achieve it.

Colleagues want him to enable married couples to combine their tax-free allowances of £6475 to boost their incomes. Yet Treasury figures claim this would cost nearly £5bn and save most -- around £2600 a year -- for high earners.

While the proposals would enable more women to stay at home to look after their children, it has been estimated the move would benefit only four out of 10 couples.

Opponents have insisted the tax break would “rob the poor to pay the rich” and help fund “ladies of leisure”.

Given the Tories’ high priority on cutting the budget deficit, which could hit £200bn this year, a senior party source has now made clear tax breaks for married couples will be unaffordable in the short term.

“This is something we can’t possibly do in the first part of a new parliament but will only happen after several budgets simply because the money will not be there,” said a member of the shadow cabinet.

While the pledge to help married couples will be in the Conservative manifesto, the senior source suggested it would be aspirational rather than a detailed commitment.

This week, Mr Cameron claimed Labour had a “pathological” opposition to supporting marriage after the party signalled that it would publish proposals arguing children’s welfare was not necessarily best protected through marriage but through “stable and lasting relationships between parents”.

Allowing married couples and gay people in civil partnerships to combine their tax allowances has been put forward by Iain Duncan Smith, the ex-Tory leader.

Last night, he also told The Herald: “I’m utterly confident David Cameron, in private and in public, has this tattooed across his heart. There may be others in the shadow cabinet who have different views but they are not the leader.”