Tony Blair took the biggest gamble of his political life yesterday and declared that Britain's destiny in Europe would be decided by the result of a referendum on the proposed EU constitution.

The Prime Minister said the poll would take place - giving the public "the final say" - after Parliament had scrutinised and approved a draft constitution he considered suitable to sign.

The referendum effectively puts his premiership on the line - with bookies already saying he is bound to lose.

Mr Blair said a "no" vote would cast the UK on to the European sidelines.

He told MPs no agreement would be found to renegotiate the constitution and announced: "The question will be on the treaty, but the implications go far wider.

"It is time to resolve once and for all whether this country, Britain, wants to be at the centre and heart of decision-making or not.

"Time to decide whether our destiny lies as a leading partner and ally of Europe or on its margins."

Mr Blair's announcement to the Commons came after days of dither and drama with leaks tumbling out of the Cabinet.

The Prime Minister had previously set his face against a referendum on the constitution, arguing it would not fundamentally alter the UK's relationship with the EU.

He restated that position again to MPs in his ten-minute statement, and gave no reason for his policy about-face.

Instead, he turned his announcement into a rallying call for all those who believed in Britain's place in Europe, castigating opponents he said had peddled "myths" and urging: "Let the issue be put. Let the battle be joined."

He told Tory leader Michael Howard: "You are not against a particular provision of the constitution, you are against it in its entirety."

None of the other 24 governments in the EU agreed with this stance, he warned.

The Tories welcomed the poll they had called for, as did the Liberal Democrat leader Charles Kennedy.

But Mr Howard mocked the Prime Minister's U-turn, reminding him that he had said last year he had no "reverse gear".

"Today we could hear the gears grinding as he came before us once again, lips quivering," said Mr Howard.

"Who will ever trust him again?"

European Parliament President Pat Cox said: "This will be a profoundly important debate for the UK and for the European Union.

"It will be a tough debate, but a winnable one."

But bookmakers were sceptical of Mr Blair's chances of winning the referendum. Ladbrokes quoted odds of 1/8 on a "no" vote, with 6/1 on a "yes" vote. William Hill quoted 1/10 for a "no" vote and 6/1 on a "yes" vote.

Mr Blair told MPs he would sign up to the proposed constitution, as long as his "red lines" on tax, social security, defence and foreign affairs and criminal justice were adhered to.

He said it was needed because of the historic expansion of the EU from 15 to 25 countries.

After negotiations, said the premier, "Parliament should debate it in detail and decide upon it. Then let the people have the final say.

"The electorate should be asked for their opinion when all our questions have been answered, when all the details are known."

No 10 said it did not expect a treaty to be published until October or November, even if it was agreed by heads of Government in June. That means that after the parliamentary process has been gone through, a referendum cannot take place until next spring at the earliest.

MPs expect the poll to be postponed until after a General Election, which could take place in June next year.

Britain is one of nine countries that have announced they would be bound by referendums on the treaty, with any one "no" vote capable of scuppering the agreement for which unanimity is required.

That might make the UK's poll redundant.

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