By Adam Fleming
BBC News political reporter
MPs will vote on Wednesday on whether to hold a referendum
MPs have rejected calls by the Conservatives and Labour backbenchers for a referendum on the Lisbon Treaty, after weeks of debate.
A majority of MPs agreed with Prime Minister Gordon Brown that there was no need for a referendum because the treaty was substantially different from the abandoned European Constitution.
All three parties promised a vote on the old constitution in their manifestos for the general election in 2005.
But the debate exposed divisions in Labour, the Conservatives and the Lib Dems.
More than 20 Labour MPs voted for a referendum, rebelling against the government. They argued that both documents were similar and the government was breaking its manifesto promise to put the constitution to a public vote.
The Conservative Leader David Cameron accused the Prime Minister of losing the courage to take the treaty to the country. A handful of pro-European Tories voted against their party's proposal for a referendum.
Now the focus is mainly on the Lib Dems. Their leader Nick Clegg had instructed his party to abstain from the vote on a referendum but 13 MPs defied him. Three members of the Lib Dem front bench resigned.
They were the Scotland and Northern Ireland spokesman Alistair Carmichael, countryside spokesman Tim Farron and David Heath, the justice spokesman.
Mr Clegg had been walking a tight-rope by calling for a wider vote on Britain's membership of the EU, rather than a vote on this treaty - a policy he inherited from previous leader Sir Menzies Campbell.
Mr Clegg argued the Lisbon Treaty was a very different document from the abandoned EU Constitution so it did not need to go to a public vote.
But many of his MPs are facing Tory and UKIP challengers in their constituencies who accuse them of breaking their promise to hold a referendum.
The Liberal Democrat leader hoped that pushing for an "in-out" vote would placate those Lib Dems who felt the public should have their say. It also meant that the party would not have to side with the Tories.
Mr Clegg was accused of papering over the cracks in his party. But when it came to the crunch, those cracks appeared to get wider.
So, how damaging is this to Nick Clegg's leadership just three months into the job?
There do not appear to be rumblings about his authority within the party. One Lib Dem MP said: "This is being seen as an early lesson for our new leader."
Mr Carmichael said he had no quarrel with Nick Clegg's leadership and that he would support him wholeheartedly from the backbenches.
Nick Clegg says his strategy showed strong leadership.