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By the genteel standards of the House of Lords, it was a particularly acrimonious debate, with coalition tensions laid bare.

It ended with a government defeat on a cross-party amendment to delay the review of parliamentary constituencies until 2018 - i.e. after the next general election.

For the first time, Liberal Democrat ministers voted against their coalition counterparts and the amendment was carried by 69 votes.

The prime minister now has to decide whether or not to try to reverse the vote in the House of Commons. With Nick Clegg having told Liberal Democrat MPs to vote against boundary changes, it does not look as if it will end happily for the Conservatives.

At stake are the Conservatives' plans to cut the number of MPs from 650 to 600 overall - from 40 to 30 in Wales, where constituencies tend to have fewer votes. It's been suggested the review could deliver 20 seats to the Tories, which could be crucial to their hopes of winning the next election.

Tory MP Glyn Davies spent an hour in the Lords watching the debate and said the result "will really put cat among pigeons". He tweeted: "Question "Will gov't reverse Lords amendment on constituency boundary reform"? 69 a big majority. Give confidence to Commons opposition."

There has been speculation that David Cameron might seek a deal with the smaller parties at Westminster, but the SNP said: "There have been no discussions and no deal will be done on the boundary review.

"Given the fall-out and petty score-settling between the Tories and Lib Dems, people will wonder if this whole process has been a waste of time and waste of money."

Plaid Cymru - whose three MPs form a joint parliamentary party with the SNP six - appears equally reluctant to deal unless the UK government offers a massive transfer of powers from Westminster to Cardiff Bay.

Even then, barring another U-turn from the Lib Dems, it is hard to see the maths stacking up. No wonder the Conservatives are choosing their candidates for 2015 on existing boundaries.

UPDATE: Downing Street say the government (the Conservative half, at least) will try to overturn the vote when the legislation returns to the Commons. A spokesman said: "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."

But comments from Deputy Prime Minister Nick Clegg suggest the prime minister will struggle to win the vote. He told the ����ý: "We are not voting them down; what we are saying is there is going to be a delay of five years.

"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."