Mr Clarke, who is one of BiE's key supporters along with the prime minister, has insisted that the group was set up to argue in favour of the European single currency.

He said that those who said it was merely a pro-European group were not telling the truth.

Referring to Tony Blair's official press spokesman Alastair Campbell, Mr Clarke said "press advisers" who "got the wind up" about the campaign and tried to play down its ambitions should be ignored.

The former chancellors' remarks at a Conservative Party conference fringe meeting are likely to be seen as a further setback to BiE.

The organisation is already beset by allegations that it has watered down its objectives in order to win Mr Blair's backing.

'No doubt about aims'

But Mr Clarke accused Downing Street of trying to down play the group's objectives because it did not want to be too seen as too closely allied with support for the single currency.

"I am in no doubt what this campaign was about and never have been," he said.

Mr Clarke says BiE was founded to promote the potential benefits of the euro

"It always was in order to campaign for putting forward the positive benefits to Britain of joining the single currency if and when the economic conditions come right, and that is the basis on which they are all coming together.

"Other things flow from that.

"The first is, it is necessary in the present climate to remake the case for the British membership of the European Union - not just we can't get out of it but why I actually personally think we should want to remain members of it and contribute to its further development, because it is in the British national interest."

In a clear reference to Mr Blair and Mr Campbell, he added: "If everybody comes on board - I won't mention names - if they come on board and their press advisers suddenly get the wind up and say, 'Don't mention the single currency, just say you're there because you're in favour of Europe' ... don't believe them."

He accused the government of being too easily swayed by press criticism, warning: "That could be one of the problems we have in Britain in Europe, saying, 'Never mind, Tony, I'm sure Alastair will sort it out.'"'

He implored: "Let us do what we believe is in the national interest."

BiE denies shift

Originally BiE declared itself to be "the campaign for British membership of the single currency."

However, critics have observed that over the summer it switched its publicly-stated objectives to promoting the benefits of UK membership of the EU and calling for Britain to maintain "a credible option to join the single currency, if the economic conditions are met".

BiE has denied that this represented a shift in position or that it had been leant on by Downing Street.

Michael Heseltine worried about the Tory Party's Eurosceptic direction

Mr Clarke also turned his fire on his own party.

Speaking shortly before shadow foreign secretary John Maples was due to outline the Tories' policy of renegotiating European treaties, he warned Conservatives against following extreme anti-European policies.

The former chancellor said opt-outs were for some a "cloak" for their real aim of withdrawal from the European Union.

Mr Clarke is the second senior figure to warn the party about the threat from Eurosceptics in its ranks at this year's conference.