David Cameron is to press on with a Commons vote to redraw the parliamentary boundaries, despite suffering a bruising defeat in the House of Lords.
Tories rounded angrily on Liberal Democrat peers on Monday night after they combined with Labour to back an amendment delaying plans for a boundary review and to cut the number of MPs until after the next general election.
Six of the seven Lib Dem frontbenchers in the upper chamber joined the revolt - the first time in this Parliament that ministers in either House have voted against the Government. The seventh, Lord Wallace of Saltaire, a Lib Dem whip who is responsible for taking the legislation through the House, did not vote. In all, 72 Lib Dems voted against the Government with none voting in favour, as the Lords passed the amendment by 300 to 231 - a majority of 69.
A No 10 spokesman said Mr Cameron still intends to hold a vote in the Commons to try to reverse the amendment. "The PM remains of the view that we should have fewer MPs to cut the cost of politics, and more equal size constituencies so that people's votes have more equal weight," the spokesman said.
However, with no overall Conservative majority in the lower chamber, the parliamentary arithmetic would appear to be against him. Experts have estimated that the planned changes, which would see the number of MPs cut from 650 to 600, could be worth 20 seats to the Conservatives at the next general election - potentially holding the key to an outright Tory victory.
However, Deputy Prime Minister Nick Clegg made clear over the summer that the Lib Dems would seek to delay them after he was forced to abandon plans to reform the House of Lords in the face of Tory opposition.
But that did nothing to lessen the anger among Conservatives, with former cabinet minister Lord Forsyth of Drumlean accusing the Lib Dems of "cheating". He pointed out that Tory parliamentary aides in the Commons had been sacked for voting against Lords reform. "You don't take the Queen's shilling and then go through the lobbies and vote against the Prime Minister," he said. "It is an absolute disgrace."
Home Office minister Lord Taylor of Holbeach said the amendment to the Electoral Registration and Administration Bill was "conceived in mischief" and motivated by "hubris and cynicism".
However, the senior Lib Dem peer Lord Rennard said the Conservatives should not be surprised at the vote. "In countries across Europe where coalition is much more the norm, it is much more normal and people understand that different parties vote in different ways on some issues while agreeing on packages of measures where they can find agreement in what they both consider to be in the national interest," he said.
There was particular anger on the Government benches that supporters of the amendment pressed ahead despite a ruling by the clerks of the House that it was "inadmissible" as it was not relevant to the Bill.