A coalition dogfight broke out today over David Cameron's frantic attempts to bring in parliamentary boundary changes by the 2015 general election.
Deputy Prime Minister Nick Clegg went nose to nose with Mr Cameron after the Tories rounded angrily on Lib Dem peers who combined with Labour to delay the review.
Putting the boot in, Mr Clegg said that it was "perfectly reasonable" for the Lib Dems to block changes, in return for the Tories' failure to meet other parts of the coalition deal.
After the bruising defeat, Downing Street maintained that Mr Cameron will still seek to overturn the amendment in the Commons - but at the moment the arithmetic seems against him.
The Lords' amendment delays the implementation of the constituency shake-up - thought to be worth up to 20 extra seats for the Tories - until 2018.
Six of the seven Lib Dem frontbenchers in the upper chamber joined the revolt, which was 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.
Mr Clegg was clear last summer that his party would try and delay the proposed changes after he was forced to abandon plans to reform the House of Lords in the face of Tory opposition.
At the time he was accused by Tory peers of "cheating" and branded "a great political sulk" - and today on BBC Radio 4's Today he mentioned the "highly regrettable personalised attack."
He added: "We are leaving the boundary changes on the statute book but we are saying that we should miss a beat and do it in the next parliament, not this one."
A Downing Street spokesman said after the Lords' vote that Mr Cameron still intended 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," he said.
Proposals range from a 1.2 per cent drop in the number of MPs in the south-east to 25 per cent in Wales.
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