Up to a third of Labour MPs may support calls for an EU Treaty referendum, says a Labour MP spearheading the campaign.
The government has said a referendum is not necessary
Ian Davidson told the BBC he believed he could persuade up to 120 MPs to support a referendum on the new treaty.
He said it was "virtually identical" to the failed EU constitution - on which a referendum had been promised.
But Foreign Secretary David Miliband said that the constitution had been "abandoned" and MPs would see the new treaty was in Britain's best interests.
The Conservatives have been calling for a referendum on the treaty, which they also say is practically the same as the failed EU Constitution - on which Labour had promised a referendum in 2004.
UKIP and the GMB and RMT unions have joined calls for a referendum.
Eurosceptic Glasgow South West MP Mr Davidson said disquiet in the Labour party was similar to that in 2004 - when former prime minister Tony Blair bowed to calls for a referendum.
He told the BBC he was confident he could persuade up to 120 MPs, who had backed a referendum in 2004, that it was still needed.
He said the strength of feeling then had persuaded the government to concede to a referendum and he believed the policy was currently in "transition".
"You have got to remember that last time the government were completely against the referendum, until they were in favour of it - and once they were in favour of it, it was almost impossible to find anyone who was against it," he said.
"The vast majority of Labour MPs have assumed that the treaty and the constitution are different.
"But once we explain to them that they are actually, in fact, virtually identical, they come back to the traditional view that they had before - and that was that our manifesto commitment has got to be honoured, that we have got to have a referendum on this constitution."
He said: "The treaty and the constitution are essentially the same thing."
Mr Davidson added that he believed the caveats and exemptions the government had said it had secured would not stand up against an "attack from the European Court of Justice".
But the foreign secretary told BBC Radio 4's Today programme that 27 European heads of state had signed a document in June saying the constitutional concept "has been abandoned".
"As Parliament gets to grips with the reform treaty that comes out come December, as they look line by line, they will see first that it's good for Britain," Mr Miliband said.
He added: "It's very different from the constitution in absolute essence and ... the 'red lines' - the key national interest in foreign policy and other areas of the United Kingdom - have been protected."
And Michael Connarty, the Labour chairman of the Commons European scrutiny committee, added: "I don't think the referendum call will be as strong as 120 and I don't think it's sustainable or necessary."