Labour leader Ed Miliband has called on disgruntled Liberal Democrats to help him block coalition plans to cut housing benefit.
He said he wants to force a Commons vote on the issue and exploit Lib Dem divisions to defeat the Government.
In an interview with the Daily Mirror, Mr Miliband urged Lib Dem MPs to defy their leader and Deputy Prime Minister Nick Clegg to "vote with their consciences".
"They are honourable people. They are in politics for the right reasons. I hope they will vote with their consciences when it comes to issues like housing benefit," he said.
His comments came after David Cameron made clear he intended to press ahead with the cuts despite warnings they would force hundreds of thousands of poor people out of big cities. And Mr Cameron insisted it was not fair that claimants lived in properties many hard-working families could only "dream of".
But Mr Miliband said: "They are cutting the housing benefit of poor people in an unjustified way, they are going to potentially make people lose their homes. That is not what they came into politics for. I hope they will vote with their consciences."
Urban MPs from across the parties have also voiced concerns over the moves, which include a £400-a-week housing benefit cap for four-bedroom homes and a 10% reduction for the long-term unemployed.
Mr Cameron stressed that the cap would still leave claimants able to receive around £20,000 a year for housing. He insisted that, although the welfare reforms were difficult, they were needed to protect other areas of spending such as the schools and NHS from austerity cuts.
The Government estimates that 21,000 households will be affected by the cap on different size homes.
Housing minister Grant Shapps said the Government intended to press on with its housing benefit cap and predicted it would lead to lower rents. Mr Shapps told BBC Radio 4's Today programme: "I don't deny that some people may well need to move. Not tens of thousands, the impact assessment says there are about 17,000 people in London whom the cap would affect. Rather than some of the catastrophic predictions that have been made, what's much more likely is that rents will start to fall."