Tony Blair and the government are awaiting the outcome of the crucial French vote on the EU constitution.
Tony Blair has been meeting Silvio Berlusconi
There has been speculation that if the French vote "No" in the referendum, Mr Blair could abandon his plans for a vote in Britain on the treaty.
Senior opposition figures say there will be no point in holding a vote in Britain if France rejects the treaty.
Polls suggest that the outcome in France, where voting started at 0700 BST, will be close.
President Jacques Chirac, leading the "Yes" campaign, has said the poll should not be a vote on his government.
The constitution aims to streamline decision-making in the enlarged EU of 25 countries. It has to be ratified by all member states to become law.
Liberal Democrat foreign affairs spokesman Menzies Campbell told The Observer it would be "ludicrous" to hold a vote in Britain without French support.
He said: "If France, which has been part of the traditional engine room of Europe, votes not to accept the treaty, there would be no point in having a referendum in the UK.
"Can you imagine trying to get the turnout up against a background of a French veto?"
Former Tory chancellor Kenneth Clarke said he could not '"for the life of me see the point" of holding a vote against the backdrop of French rejection.
In France, the governing centre-right UMP party and the opposition Socialists have urged voters to support the proposed constitution.
But there is dissent from some Socialists, as well as communists and union leaders.
If the French vote "No" Mr Blair will have to join other European leaders in talks to see whether the treaty can be salvaged.
Mr Blair would be at the forefront of any crisis as he takes up the presidency of the EU on 1 July.
The government has said Britain still expects to have a vote.
The prime minister - spending the weekend in Italy after talks with Silvio Berlusconi - will make a statement after the French result comes through on Monday.
"The pressure will fall onto our government to make very clear where they stand on this if there is a 'No' vote," said shadow foreign secretary Liam Fox.
"The government should tell us whether the process of ratification is still alive, in which case the British people must get a referendum to make their voice heard. Or they must tell us that the treaty is now dead."
France is the second EU member to hold a referendum on the document. Spain's referendum resulted in a "Yes" vote, which was then ratified by both houses of parliament.
Germany ratified the charter on Friday, with an upper house vote designed to boost France's struggling "Yes" camp.
Treaty rejectionists are thought to be ahead in the Netherlands, which will hold its referendum on Wednesday.