Sir Menzies Campbell heaps pressure on Brown by demanding poll on Europe

The Liberal Democrat leader made a surprise Uturn to withdraw his backing for the Prime Minister and call for the British people to have a say.

Sir Menzies said the public deserved an 'honest debate' about Britain's relationship with Europe.

Aides to Mr Brown dismissed the call as a 'gimmick'.

But the move left the Premier without the key support of LibDem MPs in any vote in Parliament on the issue.

Mr Brown could be defeated in the Commons as up to 120 Labour MPs, the majority of Conservatives and now the Liberal Democrats are backing a referendum.

The Cabinet is also thought to be divided with senior ministers including Jack Straw, Ed Balls, John Hutton and Alistair Darling known to harbour doubts about the treaty.

The pro-EU LibDem leader bowed to pressure from the majority of his MPs two days before his party's annual conference in Brighton.

Three days ago he used an interview in the Financial Times to say a referendum was 'not necessary'.

But 80 per cent of his front-bench and two thirds of Lib-Dem MPs, including Treasury spokesman Vince Cable, are thought to want one.

Sir Menzies's decision was seen as a desperate attempt to avoid a row at tomorrow's conference and heal his party's splits on the issue.

The LibDem leader called for a referendum on the general issue of Britain's membership of the EU, rather than a vote on the treaty itself.

But this is seen as a technicality because any vote on membership would cover the new Brussels blueprint.

Sir Menzies denied he had changed his mind to 'appease' his MPs and sidestep a row.

He added: "It's time for the political parties to end the shadow boxing on Europe.

"Let's have an honest debate on the European Union followed by a real choice for the British people.

"That means a referendum on Britain's membership of the EU. We would ask the British people the big question - whether to remain in the European Union or not."

He said the case for Europe was 'overwhelming'.

His announcement means both Eurosceptics and pro-Europeans are now calling for a national vote.

Derek Scott, of the I Want A Referendum campaign, said: "The Liberal Democrats must not try to force people into a false choice between giving even more powers to the EU and leaving altogether.

"The overwhelming majority of people in Britain want to cooperate in Europe but not give more powers away.

"The only honest thing to do is to give people the referendum they were promised on the constitutional treaty.

"Otherwise it is just meaningless."

Shadow Foreign Secretary William Hague said: "This is a clear sign of desperation from Menzies Campbell.

"The LibDems promised a referendum on the EU Constitution.

"They should concentrate on making sure this promise is kept, instead of producing illthoughtout distractions on the eve of a difficult party conference."

Sir Menzies's leadership is looking increasingly vulnerable after his party slumped to 14 points.

The latest Ipsos Mori poll found 32 per cent of voters are dissatisfied with him as leader.

A source close to Mr Brown said: "This is a bit of a gimmick.

"We had a referendum on joining the European Union 20 years ago and we are not aware that any serious party is proposing leaving the EU."

j.merrick@dailymail.co.uk