Lib Dem leader Nick Clegg has delivered a blow to those seeking a referendum on the EU reform treaty by saying his party would vote against such a move.
Nineteen Labour MPs were among those who voted for a referendum
It had been thought the Lib Dems would abstain in a Commons vote, but Mr Clegg told BBC Radio's Today: "We would vote against a referendum on the treaty."
The Tories accused the Lib Dems of "breaking a promise" on a referendum.
Despite 19 Labour MPs rebelling, the bill to ratify the treaty passed its first Commons test on Monday.
The measure will now be debated line-by-line over the coming weeks.
Prime Minister Gordon Brown signed what is now known as the Lisbon Treaty in December, but Parliament must ratify it.
'Cowardly and deceitful'
It was designed to replace the EU Constitution, which was abandoned in 2005, but on which Labour had promised a referendum.
Ministers argue that no referendum is needed as the treaty merely amends the EU's existing constitution, rather than overwriting it - as the failed constitution would have done.
The Conservatives have accused the government of being "cowardly and deceitful" for refusing to hold a referendum on a treaty which they say is substantially the same as the constitution.
And Labour rebels who oppose the treaty have warned the government it faces a much bigger revolt when it comes to the expected vote on an amendment to hold a referendum.
Mr Clegg said he agreed with the government that the treaty was "a rather different beast" to the constitution and therefore a referendum was not necessary.
Instead, he says there should be a poll on Britain's continued membership of the EU.
Lib Dem split?
"We would support the government by not voting for a referendum," he told BBC Radio 4's Today programme.
"We would vote against a referendum on the treaty and vote in accordance with our long-held position that the real referendum that needs to be had is whether we stay in the EU or not," he said.
Shadow foreign secretary William Hague said Mr Clegg's stance was "the latest instalment of the Liberal Democrats reneging on their election promise to vote for a referendum on the EU Constitution".
"They told the voters they wanted the referendum," he said.
"Then they decided to abstain on a referendum. Now they will, if necessary, bail out a failing and discredited government which is itself breaking its election promise to hold a referendum."
The Eurosceptic Conservative MP Bill Cash admitted that the rebels needed the support of the Lib Dems if they were to have any chance of wining a vote for a referendum on the treaty.
"It's obviously a matter of numbers and frankly it looks as though there aren't going to be quite enough rebels," he told Today.
UK Independence Party leader Nigel Farage said: "So obsessive are the Lib Dems in their belief in the European state that they are breaking their pledge to hold a referendum on the constitutional treaty."
In the first of at least 15 days of debate over the next month, Foreign Secretary David Miliband said the treaty was "good for Britain and good for Europe".
"The government is convinced that Britain's membership of and full engagement with the EU is good for Britain and good for Europe," he told MPs.
At the end of the rowdy five-hour debate a motion to approve the European Union (Amendment) Bill in principle won by 362 votes to 224.