Up to a third of Labour MPs could potentially support demands for a referendum on the EU treaty.
Ian Davidson is leading a campaign to reform the treaty or put it to a public vote.
He claims at least 120 Labour MPs support his campaign for an amendment or referendum, although admits support is difficult to gauge during the parliamentary recess.
Mr Davidson told the Telegraph: "On the basis of the soundings and conversations I have had with colleagues, the support for a referendum is similar to last time round when well over 120 Labour MPs publicly or privately backed a referendum."
Gordon Brown is eager to avert a widespread rebellion on the EU, which risks portraying the Labour as being split over Europe.
However, the Telegraph claims Jack Straw is privately sympathetic to calls for a referendum.
The campaigners argue not putting the EU treaty to a vote risks undermining Mr Brown's sentiments on public trust and political reform.
David Cameron today attempted to exploit this angle by accusing the prime minister of arrogance in his refusal to hold a referendum.
Writing in the Sun the Tory leader said Labour's attitude to Europe was underpinned by a "shameless arrogance", arguing this was the "big cancer eating away at trust in politics".
Mr Cameron said: "Labour's last election manifesto could not have been clearer on the EU constitution.
"It said: 'We will put it to the British people in a referendum, and campaign wholeheartedly for a Yes vote.'
"And just two months ago, Gordon Brown said: 'The manifesto is what we put to the public. We've got to honour that manifesto. That is an issue of trust with me and the electorate."
"Now he's done a massive EU-turn," Mr Cameron insisted.
The government argues a referendum is not necessary because the agreed treaty is not a constitution.
David Miliband insisted the original constitution, which was rejected by French and Dutch voters, had been replaced with a reform treaty.
Speaking to BBC Radio 4's Today programme today, the foreign secretary said: "The constitutional concept has been, quote unquote, abandoned. It has been abandoned because the treaties that were going to be brought together into a single new constitution are not happening.
"I think that as parliament gets to grips with the reform treaty that comes out from December, as they look line by line, they will see first that it is good for Britain; second that it is very different from the constitution in absolute essence, and third that the red lines, the key national interest of the United Kingdom in foreign policy and other areas, have been protected."
Nevertheless, the GMB and RMT unions are set to join the Conservatives and UKIP in calling for a referendum before the EU Treaty is set to come into force in 2009.