Labour sources confirmed on Tuesday night that the party plans to table an opposition day motion in the House of Commons stating its in-principle support for the idea, which the party adopted last week but which has long been advocated by Lib Dems.
By ruling out references to other policies, such as cutting the lowest rate of income tax from 20p to 10p, the party hopes to maximise the chances that Lib Dem MPs will vote for the motion, giving supporters a majority. Although such a vote would not change government policy, Labour hopes it will put further pressure on already strained coalition relations.
In a sign of how far Ed Miliband's party is prepared to push the issue, a party source also told the Guardian that if the government did not grant them an opposition day motion before the budget on 20 March, Labour would instead table an amendment to the finance bill, which would cause far more damage if passed.
"The Lib Dems say they support a mansion tax and we want to give them the opportunity to vote for what they say they believe in and maximise our chances of winning a vote in the chamber," said the source. "[The government] can ignore it, but if the chamber of the House of Commons show a clear political will, we hope they'll take notice of that."
A report of the Labour move emerged on the New Statesman magazine website just hours after the prime minister, David Cameron, urged the Lib Dem leader Nick Clegg not to support any opposition motion. Speaking on his trip to India, Cameron said: "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."
The sight of Lib Dems walking with Labour MPs through the voting lobby would also reignite anger on the Tory benches about how their coalition partners voted with the opposition last month to defeat a review of election boundaries, dealing a major blow to Cameron's hope of an outright victory at the next general election. Lib Dems blamed that vote on a Tory rebellion forcing House of Lords reform to be shelved.
A Lib Dem source played down what he called a "political stunt" by Labour, adding: "I'm not going to get dragged into how we react to a vote when we haven't seen it."
Tory insiders also stressed that, if Lib Dem ministers supported such a motion, it would not be a body-blow to the coalition since the party's support for the policy was widely known.
Lib Dem party members overwhelmingly backed the idea of a mansion tax on homes worth more than £2m at last year's annual conference, and the party's business secretary, Vince Cable, suggested on Sunday that there were circumstances in which he and other party members would back a Labour motion on the policy as long as it was not tied to Miliband's proposal to reintroduce the 10p tax rate.
On Monday night Clegg also suggested there were circumstances in which he would vote with Labour on the issue, telling ITV News: "Neither Vince nor I know what will be put before us so we can't of course determine in advance how we would vote. But of course the Liberal Democrats for a long time have been the leading advocate of greater fairness in tax."
Next month Liberal Democrats will use their spring conference to consider extending the mansion tax policy to a 1% levy on all major assets over £2m, such as second homes, jewellery and paintings. However, the latest proposal was dismissed by the party's treasury spokesman, Stephen Williams, who said he expected it to be "firmly spiked" by activists.
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