David Cameron on the party's tax proposals
The Conservatives say they remain committed to supporting marriage through the tax system, amid Labour claims that the plans are in disarray.
But the shadow chief secretary to the treasury, Philip Hammond, said the Tories might have to offer more limited proposals than previously envisaged.
The Tories had planned a transferable tax allowance, but Mr Hammond said it did not now look to be affordable.
He told the BBC the party is still looking at options and alternatives.
Conservative leader David Cameron has said recognising marriage in the tax system is something he feels very strongly about.
Labour has claimed the Tories' plans for tax breaks for married couples would cost almost £5bn - but the Tories said Labour had chosen the most expensive option to assess.
Meanwhile, it has emerged that a Conservative pledge made last year to increase the number of single rooms in NHS hospitals from 28% to 55% has been dropped from the party's draft manifesto.
The shadow health secretary Andrew Lansley said more single rooms were required but the Conservatives could only provide them "as resources allow".
The renewed pledges to support marriage come after a day of clashes on Sunday between Labour and the Conservatives over tax and spending plans as pre-election campaigning stepped up.
Tory leader David Cameron called Labour claims of a £34bn funding gap in his party's policy commitments "junk" as he trailed his party's health plans.
But he was forced on to the defensive over the tax breaks for married couples after telling the BBC he hoped to bring them in but could not promise them.
The Lib Dems said neither party had a convincing plan to improve the state of the UK's finances.
"There is an unholy conspiracy," the party's leader Nick Clegg said on Sunday.
"Both Labour and the Conservatives are not being straight with people about the difficult decisions required to sort out the public finances, get us out of the recession and rebuild a fairer Britain."
And the Institute for Fiscal Studies said the differences between the parties on tax and spending were "dwarfed" by the challenge facing all of them in getting to grips with the deficit.
The general election must be held by early June but is expected on 6 May, with all the main parties seeking to set the agenda after the Christmas break.