Mr Giscard D'Estaing: UK will be marginalised

Instead, Valery Giscard D'Estaing said the UK would be left on the margins, away from the main decision making.

He said Tony Blair had made a "normal choice" in his U-turn on a referendum on the EU constitution.

The EU will undergo the biggest expansion in its history when it admits 10 more countries on Saturday.

'Making Europe function'

Pressed about what actually a 'no' vote would mean, Mr Giscard D'Estaing said: "It's not a question of saying yes or no to Europe.

It's not a question of saying yes or no to Europe - it's a question of making Europe function Valery Giscard D'Estaing

"It's a question of making Europe function. Of course if someone says we do not accept the way that Europe functions, it will have to assume the consequences of its own choice."

The ex-French president. who met Tony Blair on Wednesday to discuss the constitution, compared the situation with the US, where if two thirds of people back amendments to its constitution, they are adopted.

He said if 300m out of Europe's 455m people say yes, then those who say no will have to decide "how to progress by themselves ... elsewhere".

"If finally the British say no and the other Europeans say we want to go, then they will have to find an accommodation," he told BBC Radio 4's Today programme.

"It's true in that case Britain will not be in the core of the system but in the margins of the system."

Election campaign

Michael Howard, leader of the Conservatives, said Mr Giscard d'Estaing had given the lie to Labour suggestions that a 'no' vote would take Britain out of the EU.

Mr Howard, who launched the party's campaign for the European elections on Thursday, says he wants a 'no' vote - but still wants Britain to play a leading role in Europe.

But Commons leader Peter Hain warned that rejecting the EU constitution would alienate Britain from the biggest single market in the world.

"We don't want to be on the edge of Europe - we want to be in the middle of Europe, influencing the direction of Europe and that's where this government will place us," he said.

Charles Kennedy meanwhile pledged his party would challenge "the Europhobes in the UK, those who believe that Britain would be more prosperous and safer outside the EU" in a referendum campaign.

Central role?

The Lib Dem leader used a speech in Brussels to say the "fight in Britain for positive engagement with Europe is by no means a foregone conclusion".

"In my country, as you know, the debate on Europe can be, well let's say spicy, at the very least.

"But this is not due to any entrenched Europhobia in the British people. It is because the anti-European case has been allowed to dominate by default."

Foreign Secretary Jack Straw said the UK's "central role" in the EU meant "Britain today is a confident and successful country with real influence on the world stage".

"Those national interests are threatened by a Conservative Party now driven by an anti-European cause based on a philosophy of defeatism and fear," he said.