Speaking at a closing news conference after his party's conference, the Liberal Democrat leader said: "We would vote down a Labour government Queen's Speech that was not carrying a programme that we feel deals right with this country.PR is a part of that, of a programme as a whole."
Mr Ashdown was speaking after Mr Blair suggested in Birmingham on Wednesday that the Liberal Democrat leader should "reflect carefully" on where he was positioning himself on tax rises.
The Labour leader went on to reaffirm that he was "not convinced" about PR because of the risk of giving disproportionate power to small parties.
Mr Ashdown went out of his way to emphasise the three-pronged Liberal Democrat programme of priorities, which was endorsed by the conference yesterday. He made it clear that a Liberal Democrat vote against a Labour government's first programme could also be triggered by Labour indecision on Europe, a "total failure to reform the rest of Britain's constitution", failure to tackle education under-investment, the environment or unemployment or an economic policy that was "wrong for this country".
But pressed on whether he would prop up a Labour government that was not immediately committed in its first Queen's Speech to PR, he replied: "I would not. Proportional representation is an absolutely essential part of a broad-ranging programme for what is right for Britain.
"I would not be prepared to prop up any government, or indeed vote for any government, which does not have a programme in the round which is good for this country.
"PR is an absolutely essential part of a programme for the reform and modernisation and success of Britain."
Mr Ashdown is under pressure from his party activists to demonstrate that he has not sacrificed Liberal Democrat principles by the decision to drop "equidistance" from the two main parties.
But his remarks will provoke concern within the parliamentary party, particularly among Scottish Liberal Democrat MPs who would be reluctant to oppose Labour because of its commitment to the introduction of a Scottish Parliament.
Labour, the Liberal Democrats and other parties have already agreed that a Scottish Parliament should be elected by the "additional member" system of proportional representation.
Mr Ashdown said it had been a "highly successful" conference in which the first thing that had been put in place was a commitment to a costed manifesto.
"We look forward to watching the other two party conferences and seeing whether or not they can match our clarity," Mr Ashdown said.Reuse content