The long-awaited vote on whether to hold a referendum on the EU's Lisbon Treaty is taking place.
MPs have rejected proposals to hold a UK-wide referendum on whether to ratify the EU's Lisbon Treaty.
Gordon Brown says a referendum is not needed, but some of his MPs may rebel.
The House of Commons rejected the Conservative proposal by 311 votes to 248 votes, following a six-hour debate.
David Cameron has urged Labour and Lib Dem MPs to rebel and back calls for a referendum, which he says was promised in 2005 general election manifestos.
The result means Parliament itself will decide whether to ratify the treaty, signed by Gordon Brown and other EU leaders last December.
Two Lib Dem frontbenchers have quit their roles - with more expected - to defy leader Nick Clegg's order to abstain in the vote.
Earlier, two Liberal Democrat frontbenchers quit over the party's call to abstain on the referendum vote.
Earlier, at prime minister's questions, Conservative leader Mr Cameron said the prime minister had lost his "courage" over the issue while Lib Dem leader Nick Clegg said the prime minister had "bottled it".
MPs rejected the Conservative amendment to the EU (Amendment) Bill by 63. It is not yet known how many rebels there were in the three main parties.
Mr Brown responded by saying Mr Cameron was "appeasing" eurosceptics and had put jobs at risk. He ridiculed Lib Dem plans to abstain in the vote.
All EU parliaments must ratify the treaty before it can come into force. The only country which has committed to a referendum is Ireland.
The debate and vote in the House of Commons is the key vote in the progress of the EU Amendment Bill, which ratifies the treaty, through the UK Parliament.
All three of the main UK political parties promised a public vote on the EU Constitution in their 2005 general election manifestos.
Mr Cameron says he hopes they will win, but that is unlikely unless all opposition MPs (including Lib Dems) and about 34 Labour MPs rebel against their leadership.
But the constitution was rejected by the French and Dutch electorates later that year. The Lisbon Treaty was drawn up to replace it.
He urged both parties' MPs "to keep the word they gave to their constituents" on holding a referendum at the last election.
He appealed to Lib Dems, saying "they are not part of Gordon Brown's troops. They don't have to march to his tune".
The Lib Dems pressed for a wider referendum vote but were defeated
Mr Cameron said: "If enough of them vote with us and if there are enough Labour MPs who vote to keep their word, we can get the referendum that the British people were promised."
He told Mr Brown: "We have the courage of our convictions and are sticking to that promise - you have lost your courage."
But the prime minister hit back: "If this was a constitutional treaty, we would hold a referendum. But the constitutional concept was abandoned."
Defying whip
Mr Brown's spokesman said the government remained "confident of winning the vote this evening".
Mr Clegg defended his decision to tell his MPs to abstain, and maintained that he instead wants a referendum on the "real" issue - Britain's membership of the EU.
I actually believe in Parliamentary democracy and I think this ought to be determined by Parliament Ken ClarkeConservative MP Analysis: Political fallout
But Mr Clegg faces some rebels in his own party, including Sandra Gidley, the Lib Dem health spokeswoman, who said she would be defying the party whip and voting for a referendum.
"Rightly or wrongly, my constituents will perceive that I have broken a promise I made at the last election if I do anything other than vote in support of a referendum on the Lisbon Treaty," she told BBC Radio 4's Today programme.
Foreign Secretary David Miliband denied the government feared it might lose a referendum.
Scrutiny
He told MPs in the Commons that referendums should be called "where there's a fundamental shift in the balance of power between this nation state" and the EU.
But shadow foreign secretary William Hague, who opened the debate, said: "A referendum should be held on this issue because a referendum was promised."
Ex-Labour minister Frank Field, who said he would be voting for a referendum, said: "Whether this is a constitution or a treaty, most people thought they were going to get a vote on what we are voting for today."
But ex-chancellor Ken Clarke, who said he would be voting for a poll, told the foreign secretary he risked "getting into trouble because of the deviousness and, at times, ridiculousness, of the arguments you are using".
Ex-Tory leader Iain Duncan Smith said: "People talk about the principle of letting this House decide, but the truth is it's become a little bit of a joke...
"The point about these treaties is you get one shot at this until the next time round."
'Lack of confidence'
Labour rebel Ian Davidson, who is proposing an amendment that would give a referendum on the treaty but allow the government to ask a second question, said ministers appeared not to trust the people.
All three of the main parties promised a referendum on the EU constitution in their 2005 manifestos.
But the constitution was rejected by French and Dutch voters that year and the Lisbon treaty drawn up to replace it.
The government and the Lib Dems say the treaty does not have constitutional implications, so a referendum on it is not needed.
The government and the Lib Dems say the treaty does not have constitutional implications, so a referendum on it is not needed.
It says most changes are minor and procedural and it has secured "opt-outs" where necessary.
The government says most changes are minor and procedural and it has secured "opt-outs" where necessary.
Month-long debate
But the Conservatives, some Labour and Lib Dem MPs and the UK Independence Party among others, say that it is effectively the constitution under a different name - so there should be a referendum.
But the Conservatives, some Labour and Lib Dem MPs and the UK Independence Party among others, say that it is effectively the constitution under a different name - so there should be a referendum.
All EU parliaments must ratify the treaty, which was signed by EU leaders last year, before it can come into force.
The Lib Dem leadership, which instead wants a referendum on whether the UK should stay within the EU, ordered its MPs to abstain in the Tory-led debate.
But frontbenchers David Heath, Alistair Carmichael, Tim Farron and Sandra Gidley refused to do so.
Mr Carmichael, the party's Scotland and Northern Ireland spokesman, resigned, as did Mr Farron, spokesman on rural affairs.
MPs have been debating the different elements of the treaty over the past month.
MPs have been debating the different elements of the treaty over the past month.
