The former Lib Dem leader confirmed what had already been known about the two party leaders' preparations before the election for a possible Lib-Lab government after it. In the event, the project was shelved because of the size of Labour's majority at the polls.

More startlingly, however, Mr Ashdown disclosed that he and the prime minister "jointly prepared" for forming a coalition at two points during the New Labour administration that took office in May 1997

Paddy Ashdown surprised everyone at the start of the year when announced his resignation as Lib Dem leader

Disclosing the existence of the secret plans, Mr Ashdown told BBC Radio 4's Resigning Issues programme: "It's true to say that Tony Blair and I had planned and prepared for the possibility of coalition cabinet.

"And it's true to say that the size of the Labour majority disturbed those plans.

"It's not true however to say - it may not generally be known - that all thoughts of a coalition cabinet vanished at that point.

"I said to the prime minister - incidentally before I said to anyone else except my wife - that this Rubicon of the two parties working together in government was a Rubicon I still wanted to cross.

"And I believe he wanted to cross it. And I believe it could be crossed in this Parliament.

"And on at least two occasions there were moments when that could have been done, for which we jointly prepared.

"It could have happened in the autumn of that year [1997] and it could have happened around the Jenkins proposals on proportional representation [in October 1998]."

Coalition 'sooner or later'

BBC Political Correspondent John Kampfner: Did frustration make Paddy Ashdown throw in the towel last January?

Mr Ashdown took Westminster by surprise when he announced his resignation as Lib Dem leader in January this year, just three months after the failure of the last chance for a coalition, which would have granted him a cabinet position.

He told the programme: "I do regret - not much, but I do regret - that this big thing in politics in the end could not at the moment be delivered."

But he predicted: "It will come sooner or later, I'm quite sure."

Mr Blair and Mr Ashdown had a close working relationship, viewed with deep suspicion by grassroots members of their respective parties.

Liberal Democrats feared that Mr Ashdown had been seduced by Mr Blair into giving up the party's independence.

But as the giant size of Labour's parliamentary majority became clear on 1 May 1997, MPs, members and soon-to-be cabinet ministers saw no need for allying with the Lib Dems when the mathematics so plainly did not demand it.

'Suicide pact'

Jackie Ballard MP: "Startled" by the coalition revelations

Responding to Mr Ashdown's disclosures on Tuesday, Lib Dem MP Jackie Ballard - an anti-Lib-Labber - said: "I am certainly startled."

"Perhaps one of the reasons he resigned from the leadership is that he knew there was no way he could get his party to accept a suicide pact, which is what it would have been in my view - a coalition with a majority Labour government."

Ms Ballard said a coalition would have meant collective responsibility. Yet there was "no way", she went on, that the Lib Dems would have voted for benefits cuts, student tuition fees or "budgets that don't give any more money to health and education than the Tories".

Conservative Party chairman Michael Ancram used Mr Ashdown's disclosures to attack his successor as leader, Charles Kennedy.

"Poor old Charles Kennedy," Mr Ancram said. "First we discover the prime minister has decided not to bring in PR for local elections, after Charles' desperate pleading.

"And now, Paddy Ashdown has revealed that, twice no less, he came close to selling his party down the river in a coalition.