The Liberal Democrat leader has agreed to compromise over voting reform, the issue being considered by a commission headed by Lord Jenkins, and settle for a system which would fall short of his party's previous demands.
Mr Ashdown has indicated that he would accept a diluted proportional system known as the "Alternative Vote Plus" scheme in an attempt to persuade the Prime Minister of the case for changing how the government is elected. "We must look at this issue not as purists but as political realists," he said.
His comments - another sign of growing co-operation between Labour and the Liberal Democrats - will infuriate some party members as they gather for their annual Brighton conference. They do not believe that the AV Plus system is truly proportional.
Lord Jenkins is considering proposing that around 500 MPs should be elected under AV - which means people can mark a second and third choice on the ballot paper - with a "top-up list" of 100 chosen according to their party's share of the national vote.
Many Liberal Democrat activists are already angry at what they see as their leader's "cosying up" to Labour. But Mr Ashdown said PR would "inevitably" lead to coalition governments. "We are natural partners [with Labour]," he said. "There has been a loosening up of politics, parties are now working together and co-operating."
The Liberal Democrats have always argued that elections should be conducted according to the strictly proportional Single Transferable Vote system, under which groups of MPs are elected in large constituencies, according to the proportion of votes cast for each party. But Mr Blair is "not persuaded" of the case for such a strict system.
Mr Ashdown said it would be "ludicrous" to reject an element of AV out of hand. Liberal Democrat sources confirmed that a compromise deal was expected, following discussions with Downing Street and Lord Jenkins.
The party leader made clear he would be willing to serve in a joint Liberal Democrat/Labour Cabinet. "Would I serve in somebody else's Cabinet to administer somebody else's policy? No. Would I serve in a government dedicated to the things I believe in? The answer is yes."
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