In an attempt to reassure his critics, Mr Ashdown said he saw his decision to extend the remit of the cabinet committee that includes senior Lib Dems as "the last step" of his strategy of "constructive opposition" before the next election.
Mr Ashdown's pledge came in a briefing note to MPs and party activists ahead of a tricky meeting of its federal executive last night. He also ruled out a Lib-Lab coalition before the election as "inconceivable".
His "thus far, no further" statement contrasted with Tony Blair's comment that there were "no limits" to co- operation after the two leaders extended the cabinet committee's work from constitutional reform to other policy issues.
But Mr Ashdown's internal problems were compounded yesterday when Jack Cunningham, the Cabinet's "enforcer", cast doubt on his claim that Mr Blair would hold a referendum on proportional representation before the next election. "I am not sure why he sounds more confident on that," Dr Cunningham told BBC Radio. "That has not been decided."
Cabinet sources said that while Mr Blair had not ruled out an early referendum, he would call one only if he believed he could persuade the public to back electoral reform. The betting in the Cabinet is that it will not take place before the election.
Writing in today's Independent, Mr Ashdown insists that voting reform has now been given a "fighting chance" by the Prime Minister. Defending further co-operation with Labour, he says the Lib Dems now have a great opportunity to give the case for reform momentum "by practising the kind of politics we preach".
Mr Ashdown insists he has not signed up to supporting "vast swathes of social policy". Instead, his party would try to negotiate limited, formal and tightly controlled areas in which it could work with the Government.
He told last night's meeting of the Lib Dem executive: "This is not about pulling our punches. We will oppose vigorously where we disagree with the Government."
But Mr Ashdown faced criticism over his failure to consult his party before announcing the new Lib-Lab deal last week.
Ruth Berry, a member of the executive, said: "This may be a step too far. There will be a lot of people who are very upset and very angry, particularly about the way it happened."
Although it was right to stay in the cabinet committee, she added: "I am concerned about widening into other areas such as health, education and welfare, where we have very different policies from the Labour Party."
Keith Kerr, another member of the executive, said: "I am terribly concerned for the long term future of this party and where it is leading to."
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