What could be a turning point for the party will be spelled out in a position paper in which Mr Ashdown makes clear that the Liberal Democrats will remain a "robustly independent" party with distinctive policies of its own - but one pledged not to prop up a minority Tory government in the event of a hung parliament.
Mr Ashdown's intention to scrap the policy of so-called "equidistance" was becoming an open secret, although today's official confirmation has come earlier than expected, partly because of the need to clear the air before the Littleborough and Saddleworth by-election triggered by the death last week of the Tory MP Geoffrey Dickens.
Party chiefs also judged the time to be right in the wake of the Liberal Democrats' strong showing in the England and Wales local elections. The shift towards Labour follows recent regional visits by Mr Ashdown where he was told by party workers that the pretence of neutrality between the Tories and Labour could no longer be maintained.
In an article in today's Independent, Mr Ashdown says the decision "doesn't mean that Liberal Democrats are going soft on Labour". He goes on: "Liberal Democrats are determined to change the culture of our politics - so that parties learn the habit of co-operating for the national interest where they agree, as well as competing for votes at the ballot box."
Mr Ashdown is planning a July launch of a major policy paper pinpointing the distinctions between his party and Labour. The document will highlight in particular the differing pace of proposed constitutional and environmental reforms.
Today's paper will stress in the meantime that the Liberal Democrats will compete strongly against Labour in local and national elections: "No quarter for the Tories; no let-up for Labour."
This will be evident at Littleborough and Saddleworth, where the Liberal Democrats came second in 1992 and need a 4.2 per cent swing, small in by-election terms, to win. "The most effective way to beat the Tories" will be the foremost theme of the party's by-election campaign.
Jeremy Hanley, the Tory party chairman, was quick to exploit the Liberal Democrats' shift with the argument that a vote for the party would be a vote for Labour. It was sharply criticised by Liz Lynne, the fiercely anti-Labour MP for Rochdale, which neighbours Littleborough and Saddleworth, who claimed that most Liberal Democrat MPs agreed with her.
Alex Carlile, the MP for Montgomery and a Liberal Democrat frontbencher, insisted yesterday - as did Tony Blair's office - that there was "no possibility of and no desire for" a pre-general election pact but there was a desire to talk informally. Mr Blair has already signalled, in an interview last month, his readiness to co-operate with Mr Ashdown, while giving his blessing to a Labour group promoting enhanced dialogue with the Liberal Democrats.
Mr Blair's office gave a cautious welcome yesterday. A party spokesman said: "We will wait and see how the position of their leadership and their members develop.
"We have always made clear that there is scope for co-operation and dialogue."
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