The Government is braced for a bruising clash with the House of Lords over Work and Pensions Secretary Iain Duncan Smith's flagship benefit reforms.
Ministers fear a combination of Church of England bishops and rebel Liberal Democrats could undermine a planned £500-a-week cap on benefit payments when peers vote on the measure on Monday.
Former Lib Dem leader Lord Ashdown on Sunday became the most high profile figure so far to speak out against the plans, denouncing them as "completely unacceptable" in their current form.
He said that as president of the United Nations' children's agency, Unicef, he was not prepared to support them in tomorrow's vote on amendments to the Welfare Reform Bill.
"I voted with the Government on everything until now," he said. "I see it as my job as an ex-leader to support my successor but I will not support the benefit cap in its present form."
Despite the divisions in his own ranks, however, Deputy Prime Minister Nick Clegg said he was fully signed up to the changes: "I completely back Iain Duncan Smith on this. It surely can't be fair, it can't be right, that you can be earning, if you like, more on benefits than someone going out earning £35,000."
Mr Clegg suggested there was some scope for softening the impact of the changes through "transitional arrangements" around the introduction of the cap.
However he flatly rejected an amendment tabled by the Bishop of Ripon and Leeds, the Rt Rev John Packer, which would exclude child benefit payments from the £500-a-week limit. "If you did that it probably wouldn't make much sense trying to have a cap at all," he said.
Labour, in contrast, reacted cautiously, suggesting that it would try to find a compromise between the Government and the bishops. A party spokesman daid: "Labour won't be voting against the benefits cap because we support the principles and the responsibility to take a job if you can work.
"But we will be seeking to amend the Bill, to bring a compromise between the bishops and the Government because we don't think council taxpayers should be hit with a massive bill for homelessness."