Labour and Lib Dem peers joined forces on Monday to defeat David Cameron over plans to re-draw the electoral map, leading the Tories to accuse Nick Clegg of trying to "fix the next election" in his favour.
The House of Lords voted to delay any change to number and size of constituencies from 2013 to 2018, a blow to Tory hopes of making the change before the next election. The government's plans were defeated by 68 votes.
The vote was also the first time first time Lib Dem and Tory ministers have voted against each other since the formation of the coalition in 2010.
In all, 72 Lib Dems voted against the Government with none voting in favour.
The proposed re-drawing of the map would likely benefit the Tories to the tune of 20 extra Commons seats. By siding with Labour against the plan, the Lib Dems aim to make it harder for Cameron to secure an overall majority in 2015.
Clegg decided to order his MPs to vote against the boundary changes after Tory backbenchers scuppered plans to reform the House of Lords.
Angry Tory peers rounded on Clegg and the Lib Dems during the debate in the Lords, arguing the changes to the electoral map were not connected to reform of the House of Lords - but rather had been linked to the AV referendum.
Former Conservative Scotland secretary Lord Forsyth accused the deputy prime minister of a "double cross" given Cameron had delivered Tory votes for the referendum.
"The prime minister risked the future of the Conservative Party as a party of government by agreeing to that [The AV referendum]," he said. "And yet here we have today, the Liberals still trying to gerrymander our constitution."
Tory peer Lord Dobbs slammed Lib Dem peers for voting against the boundary review, telling them he would need the "telescope at Jodrell Bank" to find their principles and accused Clegg of trying to "fix the next election".
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.
Despite the Lib Dem pledge to vote against the changes and Monday's defeat in the Lords, the prime minister has indicated he still intends to push it to a vote in the Commons.
A No 10 spokesman said last night that Cameron still intended to hold a vote in the Commons to try to reverse the defeat.
"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.
Without Lib Dem MPs, Cameron is highly unlikely to have the votes he needs to win, as he would need to convince MPs from the other small parties, including the six SNP MPs, to vote with him.
An alliance of Tories, the SNP and DUP members would number 317. There are 312 Labour and Lib Dem MPs.