Nick Clegg led his Liberal Democrat MPs through the "No" lobby to vote down a Government Bill which would have redrawn parliamentary constituencies for the 2015 general election to the likely benefit of the Conservatives.
In the deepest split yet between the coalition parties, Liberal Democrats combined with Labour and smaller parties to delay the implementation of the boundary review - thought to be worth about 20 extra seats in the Commons to the Tories - until 2018.
For the first time since the coalition's formation in 2010, Prime Minister David Cameron agreed to set aside the convention of collective responsibility and allow Liberal Democrat ministers to vote against the Electoral Registration and Administration Bill.
Mr Clegg initially supported the changes as part of a package of constitutional reforms, but announced last summer that his party would try to delay the review after the Tories forced the abandonment of plans to reform the House of Lords.
A senior Liberal Democrat source said the vote, which saw the Conservatives defeated by a margin of 292 to 334, would have no impact on the future of the coalition. "The coalition is fine," said the source. "Both sides of the coalition have known what the position is for months now. There are 101 other things the Government is doing and we will get on with those. This vote now draws a line under this issue."
But there was fury on the Conservative benches at the Lib Dem decision to vote against legislation to equalise the size of Westminster constituencies and reduce the number of MPs from 650 to 600, which Mr Clegg had previously championed as a matter of fairness.
Tory Penny Mordaunt said the Lib Dems were motivated by "spite, pettiness and self-interest" and were making "flirtatious glances" to Labour as potential coalition partners following the 2015 poll. "The Liberals have exchanged their legendary sandals for flip-flops in the hope that it will enable them to keep their options open," she said.
Amid scenes of rancour on the Tory benches directed at their Lib Dem coalition partners, Commons Leader Andrew Lansley appealed in vain for MPs to overturn a House of Lords amendment postponing the boundary changes, which he denounced as "an abuse of parliamentary process". But all 57 Liberal Democrat MPs voted in favour of the amendment, with four Tory rebels. And there was no sign of smaller parties rowing in to support the Conservatives, with only one - Naomi Long of the Alliance Party - joining them in the Yes lobby.
Mr Lansley said the delay will mean the 2015 election being fought on boundaries 15 years out of date, with "some votes counting more than others" due to disparities which have resulted in East Ham having an electorate of 92,000 while Wirral West had just 55,000. Reform would have supported "the principle of greater equality in the value of each vote" and cut £13.5 million a year from the cost of democracy, said Mr Lansley.
But shadow justice secretary Sadiq Khan said the ditching of the proposals meant that "selfish and partisan" changes had been "stopped in their tracks by peers and MPs from all sides". Mr Khan said: "David Cameron should try to win elections fair and square and not by moving the goalposts. It would have been an insult to democracy to reduce the number of elected MPs by 50, while over 100 new unelected members of the House of Lords have been created since the last election with another 50 reportedly due very soon."