David Cameron has said he would be "disappointed" if Nick Clegg voted in favour of a mansion tax on expensive homes.
The Liberal Democrat leader has not ruled out joining Labour in a Commons vote to support the levy on properties worth more than £2 million proposed by Ed Miliband.
The mansion tax was a flagship policy in the Liberal Democrats' 2010 manifesto, but is bitterly opposed by Conservatives and did not feature in the post-election coalition agreement setting out the Government's programme.
The Deputy Prime Minister has already clashed with Chancellor George Osborne over the issue, as the row threatened to drive a rift between the coalition parties - already taking pot-shots at one another in the campaign for the February 28 Eastleigh by-election.
Mr Osborne has dismissed Labour's policy as a "tax con" which would see state inspectors assessing the value of homes across the country.
But, in an interview for ITV News, Mr Clegg retorted: "The Liberal Democrats have always been unambiguous that they want to make the tax system fairer. The Conservatives don't want to do that. They don't want, perhaps, to offend people in very large mansions.
"They need to answer for themselves. I'm absolutely sure that what we stand for is the right thing."
Asked whether Mr Clegg had told him which way Lib Dems were planning to vote, Mr Cameron told ITV News: "I haven't asked him the question, but as it's not in the Coalition Agreement to have a Mansion Tax, I would be rather disappointed if he did."
Speaking during his visit to India, Mr Cameron made light of suggestions that the Labour leader's initiative might tempt Mr Clegg into a future coalition with Labour.
"That's a matter for him to answer," the PM told Channel 4 News. "It's enough running one coalition, without trying to work out what another one would be."