His announcement follows the prime minister's speech on Tuesday in which he promised to lead the Britain in Europe campaign.

Mr Blair claimed the Tory position risked taking Britain out of the European Union completely, while he described the party's policy on the euro was a "fatuous fudge".

Writing in The Times, Mr Clarke said: "To the embarrassment of both of us we are in broad agreement on the big issues of Europe.

"That is why I am pleased that in his speech he made it clear he will give his active support to the Britain in Europe campaign.

Former Chancellor Ken Clarke backs the euro, despite its falling value

"I will also actively support that campaign which will be conducted by all of us on a non-partisan basis."

Mr Clarke added: "Tony Blair and I share the view that Britain has gained enormously from its membership of the European Union and that Britain must maintain a credible opt-in to join the single currency - but only when the economic conditions are met.

The former Conservative Chancellor, Kenneth Clarke, defends his actions

"This is the mainstream common sense view of the majority of people in this country."

Only "fanatical anti-Europeans" could argue that Britain should close off forever the option of future membership, he said.

Hague: Hardly shock news

Conservative leader William Hague attempted to laugh off Mr Clarke's move back into the limelight on the euro.

"It's hardly shock news that Ken is in favour of the single currency," he told BBC Radio 4's Today programme.

Conservative leader William Hague, talking to the BBC's Today programme

He acknowledged there would be differences of opinion on the euro across parties, but said the options had now become clear to the electorate.

"If people vote Labour at the next election, they're voting to get rid off the pound, if they vote Conservative they're voting to keep it.

"My policy is not extreme unless the vast majority of people of this country are extreme and I don't believe they are."

Mr Hague added that he believed the single currency would be a crucially important and defining issue at the next general election.

'I'm not embarrassed by Ashcroft'

The Tory leader also insisted he was not embarrassed by the row over his party's treasurer, who is its biggest single funder, a UK tax exile and the UN ambassador for Belize, where he is also a national

"I'm not embarrassed to have somebody as my treasurer who is making sure we broaden the funding base of the party," Mr Hague said.

"I'm not going to be pushed into saying somebody is guilty of something when nobody has proved that they are.

"Belize is a friendly country - this is a person who is entitled to vote at the general election."