David Cameron has been accused of acting like a "schoolboy bully" in the row over planned cuts to tax credits as a constitutional crisis loomed over the prospect of the House of Lords throwing out the changes.
In a vote on Monday peers could decide to kill off the measure, forcing the Government to think again, even though the controversial package of cuts has already been approved by the elected House of Commons.
The Prime Minister is reported to be considering packing the Lords with hundreds of new Tory peers to prevent future defeats in the upper chamber but Liberal Democrat chief whip Lord Newby mocked Mr Cameron's tactics.
He said: "The Prime Minister is acting like a schoolboy bully. He has been challenged in the playground so he is threatening to bring round lots of his mates to duff us up. He will be creating a constitutional crisis.
"But that will be a crisis of his creation, not ours."
The Liberal Democrats have tabled a so-called "fatal" amendment which would block George Osborne's cuts, which are due to take place in April. A Labour amendment would also block the measures and calls for the Chancellor to put in place a transitional scheme to help low-income households cope with the changes before they can be considered again.
Lord Newby told BBC Radio 4's Today programme that Mr Osborne's plans faced defeat on Monday and " the Government has to think again".
The Government does not have a majority in the Lords and an alliance between the opposition parties and crossbench peers would result in a defeat for ministers.
But former head of the civil service Lord Butler, a crossbencher, warned peers not to set up a confrontation with the Commons over the measure.
He told Today: "The House of Lords has long accepted the supremacy of the House of Commons on tax and expenditure, however frustrating that might be for members of the Lords. There is no doubt that this is a tax and expenditure issue. It is a central plank of the Government's cut of £12 billion in welfare on which they were elected so recently.
"In my view it would be quite wrong for the House of Lords to overturn it or say that they will only consider it if the Government will do this or that."
The former cabinet secretary said it would be "very damaging" if Mr Cameron called for Tory reinforcements in the Lords as it was "already too big", but the Prime Minister would have a "legitimate grievance" if peers blocked the tax credits motion.
"The fact is that this was established 100 years ago that the House of Lords doesn't oppose the House of Commons on tax and financial matters.
"The Government would have a quite legitimate grievance if it did. It would be really an example of the House of Lords getting too big for its non-elected boots."
The Liberal Democrat "fatal" motion appeared unlikely to succeed after it was confirmed that Labour peers would not be instructed to vote in favour of it.