But before setting off to meet with his local party Mr Ashdown insisted the close ties he and Labour leader Tony Blair had formed between their two parties were here to stay.

Paddy Ashdown interviewed on Today: "The co-operation is not just my policy, it is the Liberal Democrats' policy"

The prime minister backed him, saying the Lib-Lab co-operation would continue - and was supported by the public - despite Mr Ashdown's departure.

Mr Ashdown warned that any Lib Dem MP who failed to support his co-operation deal with the government was unlikely to win the party leadership, and insisted the "constructive opposition" he had forged with the government was now embedded in his party.

Most of the "runners and riders" who might succeed him backed his policy, he said.

Potential leadership contender Nick Harvey backs the close co-operation with Labour

"The chances of anybody standing who will succeed who goes against what has been the established democratic will of the party, I don't think is terribly good," he told BBC Radio 4's Today programme. "But in the end that is for the party to decide."

"I am confident that the policy we have followed is no longer Paddy Ashdown's policy, it's the Liberal Democrats' policy. It's been democratically endorsed by them. It is now firm, it is clear."

He denied his resignation announcement, which included a six-month notice period, had immediately sparked a battle for succession.

"I suspect that you will find that every one of those who'd like to succeed me knows very well indeed that the one thing that will most damage their leadership possibilities in the future is to begin to run before there's even a vacancy," he said. "I'm running this party up until June."

Potential candidates should pull together in the coming months before he stands down to achieve the best possible results in the upcoming series of local, Scots, Welsh and European elections.

Battle for succession

Meanwhile leadership candidates - a number of whom have made no secret of their unhappiness at the Ashdown-inspired closeness with Labour - have, gently but unmistakeably, begun to jostle for his job.

Charles Kennedy is one of the chief potential contenders

One almost certain candidate, Nick Harvey MP - seen as Mr Ashdown's own chosen heir - put in an early marker, backing the pact with Labour.

He insisted that although Mr Ashdown's departure would inevitably alter the two parties' close co-operation, it would continue.

"I don't think it can continue in exactly the same way - the personal chemistry is obviously a considerable part of it - but there's no reason at all why the dialogue won't continue because there's a compelling logic to it," he told the same programme.

He acknowledged there had been "a lot of anger" within the party - notably at the grassroots - at the way the extension of co-operation was announced before Christmas, but MPs and the party executive had accepted that it was "the right way forward".

Other chief contenders are Lib Dem health spokesman Simon Hughes, agriculture spokesman and former party president Charles Kennedy, foreign affairs spokesman Menzies Campbell and Treasury spokesman Malcolm Bruce.

Prime Minister Tony Blair insists the Lib-Lab ties will survive Paddy Ashdown's resignation

Chief whip Paul Tyler and frontbenchers Matthew Taylor and Don Foster are also seen as possibles. Deputy leader Alan Beith - who fought Mr Ashdown for the leadership in 1988 - has already ruled himself out.

Blair claims public support for ties

Tony Blair said the public did not want to see political parties "stuck in rigid tribal boundaries".

"I've no doubt at all that co-operation will continue, for two very good reasons," he said. "The first is that there are common areas of agreement - some shared political heritage between the Labour Party and the Liberal Democrats and we shouldn't be afraid of saying so."

"But the second reason is there's a profound change in the public spirit of these things. People want a different party politics today. They don't want parties stuck in rigid tribal boundaries."