Michael Heseltine explains his reasons to the BBC

He is the second senior Tory figure to throw his weight behind Tony Blair's campaign to boost Britain's involvement in Europe.

The former deputy prime minister has joined ex-Chancellor Kenneth Clarke in backing Mr Blair's decision to lead the pressure group Britain in Europe when it launches in the autumn.

Michael Heseltine: The time is right

"I have always made it clear that the only way in which this campaign can be properly conducted is on an all-party basis," he told BBC Radio 4's The World this Weekend.

"When I felt that such an alliance had been established under the leadership of the prime minister, then I would myself join it."

Call for euro referendum

He said it was obvious that "nobody" thought Britain should now ditch the pound.

"But we have to start preparing so that when the conditions are met we can take that step."

He called on Mr Blair to hold the referendum on joining the euro before the next general election.

"I think we have to end this uncertainty."

Mr Blair has said he intends to examine whether five key economic tests have been met early in the next Parliament, and if so call the referendum.

But reports have repeatedly suggested the referendum could be postponed for several years.

Blow for Hague

Mr Heseltine's views on Europe will come as no surprise to Tory leader William Hague - but his backing of Mr Blair does little to boost the image of a united Conservative policy for Europe.

His comments follow Mr Clarke's pledge to fight alongside Tony Blair in the new pro-European alliance earlier last week.

Writing in The Times, he said: "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.

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

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.

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."