They believe that questions over the Prime Minister’s future will inevitably cloud a campaign which many voters will regard as their last and best chance to oust him.

Mr Blair, due to sign a provisional treaty on the European Constitution later this month, is committed to calling a referendum on ratifying the document within two years.

Although Labour is still expected to win a general election next year, party strategists acknowledge tthis could be with little more than a third of the vote. Polls suggest that the fallout from Iraq has created an anti-Blair majority among the British public.

“Everybody knows that after the election the referendum will be the next crunch point for all of us,” said a No 10 source. “Tony has always wanted a pro-Europe vote to be his legacy. Our problem is that the referendum could now be about him, rather than Europe.” Before Mr Blair’s recent announcement that he intends to stay on until 2009 or even 2010, many ministers assumed that he would quit shortly after the referendum. Some of them are worried that his promise has made it more likely he will lose the vote on Europe, as well as his job.

Aides note that other EU prime ministers have not had to resign after being defeated in such referendums, but also accept that he would find it hard to continue if he lost to a campaign based on the theme of “vote ‘no’ to get Blair out”.

A number of pro-Europeans have recently told the Prime Minister that he should not seek to head the “yes” campaign. These include Charles Kennedy, the Liberal Democrat leader, who hopes to capitalise on anti-war sentiment among Labour voters at the next election.

Polling research published last month by the Foreign Policy Centre has also suggested Mr Blair should be kept in the background during the referendum campaign because he risks turning off key swing voters.

Instead, the centre’s report said that Chris Patten, the former EU commissioner, should head the campaign with a supporting cast of Gordon Brown, Kenneth Clarke and Mr Kennedy.

Mr Brown has been privately sceptical towards some aspects of the EU constitution and there are fears that the Chancellor might “sulk in his tent” during the referendum — not least because defeat for Mr Blair could re-open the prospect of Mr Brown taking over earlier than expected.

But the Chancellor’s allies say he neither wants to be seen as a “wrecker”, nor pick up the pieces of a referendum defeat which could have profound consequences for both party unity and Government policies on the EU.

Lucy Powell, director of the Britain in Europe group, said: “We hope to have a cross-party campaign. It will be really important that people focus on the real issues rather than use it as an opportunity to kick Tony Blair.”

One source said: “I don’t think we would hold a referendum during the UK’s EU presidency in 2005, but we may also want to wait around and see what happens in some of the other eight member states having votes on the constitution.”

There have been suggestions that the French referendum, due to be held in the second half of 2005, could bail Mr Blair out of his difficulties by delivering its own “no” verdict.

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