Jackie Ballard, the Liberal Democrat MP for Taunton, was one of four members of the executive who opposed the move in a 15-4 vote at the Commons, but Mr Ashdown comfortably secured the backing he wanted. Another meeting of the executive, on 7 December, will discuss calls for a national ballot of members on the plan.
Mr Ashdown had promised the party before the meeting that he would not edge the party any closer to Labour before the next general election.
In an attempt to reassure his critics, Mr Ashdown said that he saw his decision to extend the remit of the cabinet committee that includes senior Liberal Democrats as "the last step" of a strategy of "constructive opposition". Mr Ashdown's pledge came in a briefing note to MPs and party activists.
He also ruled out a Lib-Lab coalition before the election as "inconceivable".
His 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.
Earlier yesterday, Jack Cunningham, the Cabinet's "enforcer", cast doubt on Mr Ashdown's 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 Liberal Democrats now have a great opportunity to give the case for reform momentum "by practising the kind of politics we preach". He told last night's meeting of the Lib Dem executive: "We will oppose vigorously where we disagree with the Government."
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