MPs will today vote on whether the EU treaty should be put to a public vote.
A Conservative amendment calling for a referendum on the Lisbon treaty goes before the Commons today, with the Tories hoping they can attract enough Labour and Liberal Democrat rebels to force a plebiscite.
The government has refused to put the treaty to a public vote, insisting it is substantially different to the original EU constitution, upon which Labour did promise a referendum in its 2005 manifesto.
The Conservatives, however, claim Gordon Brown is going back on his word and are pushing for a referendum, as was also promised in their own manifesto.
The Liberal Democrats have been told to abstain from tonight's vote, with leader Nick Clegg branding it a "smokescreen" distracting from the real issue of Britain's future in the EU.
Mr Clegg will, however, face a rebellion, with likely rebels expected to include members of his own front bench.
Lib Dem justice spokesman David Heath is among those planning to vote in favour of an EU treaty referendum.
David Cameron appealed to Lib Dems to vote with the Tory amendment, reminding MPs they are "not part of Gordon Brown's troops. They don't have to march to his tune".
The Tory leader 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."
Mr Brown faces a larger rebellion, with many Labour MPs unhappy with the denial of a referendum, but no-one in the government is expected to vote against him.
More Labour MPs could be tempted to rebel if an amendment led by Labour backbencher Ian Davidson is called rather than the Conservative motion.
Last night the Commons firmly rejected Lib Dem calls for a referendum on Britain's role in the EU, voting against their amendment 471 to 68.
Mr Clegg accused the Conservatives and Labour of trying to "deny the people of Britain a real say over our future in Europe".
He said the Westminster "establishment" was "terrified" of a public debate on EU.