Parliament began a lengthy process of scrutiny of the European Union reform treaty on Monday with members of Prime Minister Gordon Brown's own Labour party vowing to force him to hold a referendum.
The process began with a heated debate in parliament and will continue in parliamentary committees for six weeks.
Supporters of a referendum had hoped to embarrass Brown by voting on an amendment on Monday calling for a national plebiscite but the speaker decided no vote would be held on the rebel amendment.
Critics accused the government of stifling discussion on the treaty out of fear of losing the argument.
Parliamentary support for a referendum is insufficient to defeat the government as things stand, but the persistent calls for a vote in coming weeks could prove uncomfortable for Brown.
They will also focus the spotlight on Britain's ambivalence over the EU at a time when he is trying to convince European leaders that he is just as committed to the EU as his predecessor Tony Blair.
Supporters of a referendum hope that public momentum will grow, aided by some national newspapers which also back a vote.
"This is the start of quite a long campaign," said backbench Labour MP Jeremy Corbyn, who backs a national vote.
"All parties fought the last election with the manifesto commitment to hold a referendum, we put one in the Labour manifesto, so it is our duty to hold one," he added.
All EU governments must ratify the treaty, which was signed in Lisbon last month and which replaces the EU constitution rejected by French and Dutch voters in 2005.
Blair had promised a national vote on that constitution but the government argues the new treaty does not impinge on British sovereignty and therefore requires no referendum.
Some Labour lawmakers and most Conservatives, however, argue there is little difference between the two documents.
If a vote is held, the government could potentially lose, given the British public's uneasy relationship with Europe and hostility towards the EU seen in some national newspapers.
The government says the treaty does not substantially change Britain's relationship with Europe but parliament's Foreign Affairs Committee said on Sunday that key parts of the Lisbon treaty were the same as the rejected constitution.
The bi-partisan committee accused the government of downplaying the importance of new foreign policy institutions established by the treaty.
Open Europe, a think-tank, echoed the committee's conclusions, saying on Monday that the Lisbon treaty is almost identical to the rejected constitution.
"Quite apart from the fact that the government promised us a referendum, the public must be given a say on this because it will mean a huge transfer of control over everything from the health service to the economy, and from migration to the rights of criminal suspects," said Open Europe director Neil O'Brien.
After scrutiny by committees, the treaty returns to the Commons for another vote before heading to the House of Lords, where it could also face opposition.
