Budget 2012: Liberal Democrats demand new mansion tax ahead of Budget

Ed Davey, the Energy Secretary, broke the Coalition truce to make the case for taxing the “assets, land and property” of the wealthy more intensively just five days before the Chancellor’s statement.

Mr Davey, the newest Lib Dem in the Cabinet, said he knew the Conservatives would “have a problem with that” but declared: “Conservatives have their priorities... and we have ours.”

He intervened as the Conservative and Lib Dem leaderships struggled to agree details of the Coalition’s third Budget, which George Osborne is due to deliver in the Commons on Wednesday.

Labour said the pre-Budget dealings inside the Government had descended into “a shambles”.

After returning from their trip to the United States, David Cameron and Mr Osborne held a conference call yesterday with Nick Clegg, the Deputy Prime Minister, and Danny Alexander, the Lib Dem Chief Secretary to the Treasury, in an attempt to reach agreement.