Fewer than a third of voters have firmly made up their minds whether to back Britain's continued membership of the European Union, according to a poll published 40 years after the last referendum on the issue.

Seven out of 10 remain open to persuasion by the time of the vote, which the Conservative Party's general election manifesto promised will take place by 2017, a survey of almost 4,000 people for thinktank British Future found.

But it suggested that some of the most passionate advocates of the In and Out causes might actually put off more people than they persuade.

Almost six out of 10 (59%) of those questioned by pollsters Survation said they distrusted europhile former prime minister Tony Blair when he speaks about the EU, while 56% said they did not trust Ukip leader Nigel Farage on the issue. Just 28% said they trusted Mr Blair and 36% Mr Farage on the issue.

In 1975 it was a Labour prime minister - Harold Wilson - meeting a manifesto promise to let the public decide whether to remain a part of what was then the European Economic Community, into which Edward Heath had led the country two years previously.

Like David Cameron four decades on, he set out to renegotiate the terms of membership to persuade voters to back "in" - which eventually prevailed comfortably by just over 67%.

British Future director Sunder Katwala said the poll suggested that the In and Out campaigns should be wary of giving a prominent role to those keenest to lead them.

Mr Farage, in particular, "appeals to existing supporters but repels the majority of British citizens who remain undecided about Britain's place in Europe", said the thinktank.

The survey found that Mr Blair - who intervened in the recent general election campaign with a speech warning of the dangers of sleepwalking into "Brexit" - is distrusted by the public whether they are "in" or "out", "decided" or "undecided" alike.

Some 49% said they trust Prime Minister David Cameron on the EU, against 41% who do not. European Commission president Jean-Claude Juncker polled badly, distrusted by 51% of the public and trusted by only a quarter (25%).

Just 16% of those questioned said they were definitely in favour of staying in the EU and 12% definitely in favour of leaving. More than half (59%) said they were "leaning" in one direction or the other - 31% towards staying and 28% towards leaving - but would want to know the final conditions of any deal struck by Mr Cameron with Brussels before making their minds up.

Another 13% said they do not know which way they will vote.

Mr Katwala said: "The EU referendum is wide open. With seven out of 10 people wanting to hear more before they make up their minds - the 'Don't Knows' and the 'Leaners' will decide whether Britain stays in Europe.

"Either side could win this referendum. Their problem is that the most prominent and passionate voices could help them to lose.

"Both Nigel Farage and Tony Blair appear to be rather more likely to harm their cause than to help it, when preaching beyond the already converted.

"This research suggests that both sides of the referendum campaign might want to be wary of those who are keenest to lead the charge for them."

:: Survation questioned 3,977 adults online across Great Britain from May 8-14.

Downing Street played down reports suggesting that the referendum could be held as soon as next May, insisting that the Prime Minister has not taken a decision on the date.

Mr Cameron's official spokeswoman said: "The PM's view on the timing of the referendum hasn't changed, which is that the latest it will be is the end of 2017. If It can happen earlier, he is open to that."

The Daily Mail reported that Mr Cameron had overruled advice from the Electoral Commission that the vote should not be held on May 5 next year because it would clash with other elections in England, Scotland and Wales.

Asked about the Prime Minister's view on the Electoral Commission's advice, the spokeswoman said: "He has not taken a view on when the referendum should be."