Paddy Ashdown: "If it has to be after the next General Election then so be it"

Until now, the Liberal Democrats have insisted that the ballot - which will offer voters a choice between the existing first-past-the-post and the alternative vote top-up systems - had to take place in this Parliament.

Tony Blair: In charge

But speaking on the BBC's Breakfast with Frost programme, Mr Ashdown indicated he was now willing to be more flexible if Tony Blair thought it was necessary to postpone the poll.

"If because of reasons beyond our control it has to be after the next election, then so be it - but there must be a good reason for that," he said.

Mr Ashdown defends the holding of a referendum

Mr Ashdown told the BBC that his party and Labour had an agreement that the referendum would take place before the next election.

"That is the position that the government has agreed to and I haven't seen any evidence that the government is delivering in bad faith," he said.

But he also said he did not want to "close off options" and it was right that voters should be able to assess the plans in the context of other far-reaching constitutional reforms, particularly to the House of Lords.

Mr Ashdown said he thought Mr Blair was "emotionally and intellectually" persuaded of the case for reform, and that he would eventually gain considerable support from Labour MPs.

"He stakes his position and he brings people to it slowly," said Mr Ashdown.

Labour promise

The Labour Party made a commitment to hold the referendum in its election manifesto.

But because the Cabinet is known to be deeply divided on the issue of voting reform, some commentators say a delayed referendum would avoid the splits becoming public for a while, and that Mr Ashdown's remarks could be interpreted as a move towards accepting the inevitable.

But postponement could undermine Labour's pact of co-operation with the Liberal Democrats on constitutional issues.

It could also put Mr Ashdown, who has effectively staked his position on winning a change sooner rather than later, under considerable pressure from his own party members.