MPs will vote at 1900 GMT on whether to hold a referendum
Gordon Brown has clashed in the Commons with Tory leader David Cameron and Lib Dem Nick Clegg over an EU referendum.
Mr Cameron said Mr Brown had lost his "courage" while the Lib Dem leader said the prime minister had "bottled it".
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 at 7pm.
As MPs debate the issue, Lib Dem Tim Farron told the BBC he would quit his frontbench role and back a referendum.
There are reports that more Lib Dem MPs are preparing to defy the order to abstain in the long-awaited vote on whether there should be a referendum on the Lisbon Treaty.
The Tories say all parties promised a referendum on the EU Constitution and say there should be one on the treaty as it is mostly the same.
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.
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.
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."
Mr Brown's spokesman said the government remained "confident of winning the vote this evening".
Liberal Democrat leader Nick 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.
A debate on whether to allow the Lib Dem amendment on Tuesday ended in heavy defeat - 68 MPs backed the call, 471 voted against it.
But Mr Clegg faces some rebels in his own party with Sandra Gidley, the Lib Dem health spokeswoman, saying 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.
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.
Mr Miliband cited the referendums on Scottish and Welsh devolution as examples of where constitutional change had made a referendum necessary.
HAVE YOUR SAY
Let's have an open debate about what the treaty means to this country
Mark Taylor, Leicester
But shadow foreign secretary William Hague, who opened the debate, said: "A referendum should be held on this issue because a referendum was promised.
"It was promised by the government, it was promised by the opposition and it was promised by the Liberal Democrat party as well."
And 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 and pro-European MP Ken Clarke said that while he would be voting with the government, he did not agree with Mr Milliband's arguments against a poll.
He told the foreign secretary he risked "getting into trouble because of the deviousness and, at times, ridiculousness, of the arguments you are using".
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 the government appeared not to trust the people.
"One of the real reasons why we're not having a referendum in Britain is because they don't have confidence in the people producing the right result," he said.
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.
It says most changes are minor and procedural and it has secured "opt-outs" where necessary.
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.
MPs have been debating the different elements of the treaty over the past month.