Mr Blair issued a passionate appeal for cross-party support for the campaign in a keynote speech on Tuesday.

John Sergeant: "On the single currency Mr Blair wants to keep his options open"

He also launched a blistering attack on the Conservatives, calling them extremists for their position on Europe and the single currency.

Mr Blair said: "I will join men and women of all political persuasions and none and give my active support to the Britain in Europe campaign when it is launched in the autumn."

The case for Europe

The prime minister added: "Once in every generation the case for Europe needs to be restated, I believe in it and I will give leadership to it.

"The case that we make is exactly that - for Britain in Europe - and I urge people across all parities who oppose extreme Euroscepticism which threatens exit from Europe to campaign for Britain in Europe."

The Conservative Party and some sections of the media were moving from a position of Euroscepticism to holding a position which was simply anti-European, he said.

This he said bore "no logic" and was "damaging to Britain's interest and must be resisted".

Mr Blair described Mr Hague's position on the euro as a "fatuous fudge" and said to have the option of joining the single currency it was necessary to prepare.

Tories 'ready to leave' Europe

He compared what he said was the extreme anti-Europeanism of the present Conservative Party to the extremism of Labour in 1983, when the party was hostile to Europe.

Mr Blair said when the Conservatives called for the cancellation of plans for preparing for the euro and the re-negotiation of Britain's membership of the EU they were really code words for "being ready to leave".

He said following the Tory path would lead the UK "either out, or back to the margins, of Europe".

The prime minister called on the opposition to "come to their senses" and put what he said is the national interest first.

Mr Blair's remarks come after Labour was beaten by the Conservatives in June's European elections.

The Tories won the elections on a platform of keeping the pound, at least in the medium term.

Responding to the speech for the Tories, Shadow Chancellor Francis Maude said that the prime minister was trying to drag the UK into the euro by "stealth".

He said most people backed the Conservative position on Europe. This was: "In Europe, but you don't be in the euro, you don't have to scrap the pound."

No change on euro

During the speech Mr Blair did not redefine his government's position on the euro.

Addressing media speculation over changes in policy Mr Blair said simply: "If the policy changes, I'll tell you." He added it had been the same since Chancellor Gordon Brown set it out in 1997.

And that the once the UK's economic conditions for joining the single currency are met, a referendum will be held early in the next parliament to let the public decide on entry.

Mr Blair added: "If we don't prepare we close the door on the option of joining."

Turning to defence and foreign policy he said it was natural that the EU should move towards further co-operation on these issues.

He added: "We need better, fuller European defence co-operation."

He said Labour's policy of "constructive engagement" had helped the UK in practical ways and had helped the restore Britain's influence in the EU.