Tony Blair has lost the authority to persuade voters to back the European constitution, according to a large-scale study of public opinion.
Labour supporters are less likely to back the constitution if they are told the Prime Minister is endorsing the treaty, according to the research by academics responsible for a study of attitudes in the run-up to the general election.
Pro-Europeans said they feared that the referendum could turn into a vote of confidence in Mr Blair.
The team from Essex University released the findings from the British Election Study, which has charted the public mood in every general election since 1964. They found that 41 per cent of Labour supporters from a sample of voters would back the EU constitution when asked a neutral question about the treaty.
But when the people in the sample were asked whether they would back it if Mr Blair had called for a "yes" vote, support slumped to 32 per cent. Support was also down to 36 per cent when a third question was put asking if people would back the constitution if it was endorsed by Mr Blair and his Labour Party.
By contrast a similar study carried out in the run-up to the 2001 election found backing for joining the single currency jumped from 33 per cent to 44 per cent among Labour supporters if voters were told the Government backed the move. Across all voters, the survey found 24 per cent of voters in favour of the constitution, compared with 37 per cent opposed and 34 per cent undecided.
Paul Whiteley, professor of Government at Essex University, and a co-author of the British Election Study, said the finding showed Mr Blair had lost authority. He said: "It is now very doubtful if the constitution can be sold to the British people."
Denis MacShane, the former minister for Europe, warned against a referendum campaign becoming a vote of confidence in Mr Blair. He said: "I have always said that any referendum campaign has to be based on information and must not be focused either on one individual or one party.
"Eddie Izzard, Sir Digby Jones, Brendan Barber will be as important in the campaign as Charles Kennedy, Tony Blair, Ken Clarke and Michael Heseltine. If we come to a campaign, every minister must start to make the case for Europe. It can't be a one-person show."
Lucy Powell, the campaign director of the pressure group Britain in Europe, said the Prime Minister had to lead a campaign for a "yes" vote, but emphasised that campaigners needed a broad base of support. She said: "It has been a long time since Tony Blair alone would be able to persuade the majority of the public to back something.
"A lot of key figures can persuade different members of the public to vote, including Gordon Brown, Ken Clarke, Charles Kennedy as well as Tony Blair and trade union leaders."
Sir Menzies Campbell, the Liberal Democrats' deputy leader, said: "These results reflect the fact that so far the case for the constitutional treaty has not been properly put. Too much ground has been given to the 'no' campaign. It will only be regained by a broad-based, unified, positive campaign in favour."
Gordon Brown attempted to allay fears over threats to Britain's annual £3bn rebate from the European Union as he insisted that the Government would veto any attempt to remove it.
The Chancellor made it clear that the rebate was non-negotiable as Jack Straw and other European foreign ministers spent the day in budget talks behind closed doors in Brussels.
Mr Brown said: "It is wholly justified and if we did not get the result that we wanted, we would not hesitate to use our veto."
Britain's threat to use the veto sparked fury in Brussels last night. Sweden's State Secretary, Lars Danielsson, said Britain was a "a minority of one" and had to accept the rebate could not continue for ever.
But Mr Straw pointed out that the European Commission was proposing a 35 per cent increase in EU spending under a new financial package for 2007-13. He went on: "That is completely unacceptable. No national government would ever propose an increase of this level."Reuse content