In an interview on the BBC's Breakfast with Frost, he said any delay in announcing a referendum on the issue would be a breach of Labour's manifesto commitments.

His comments come four days before Lord Jenkins' commission on electoral reform is due to deliver its report.

Earlier this week, prospects for reform in this parliament appeared to have receded when Leader of the House Margaret Beckett said manifesto commitments were simply a "declaration of intent and goodwill".

She said they were not necessarily a list of what the government would deliver in one parliament.

Lord Jenkins' report out on Thursday

But Mr Cook said he saw no reason why a referendum would not go ahead in this parliament.

He said: "Our manifesto did make a commitment to a referendum in this parliament and at the present time I have no reason to believe that we are not going to have a referendum in this parliament."

Lord Jenkins is expected to recommend a hybrid system of proportional representation known as "alternative vote plus".

Around 500 MPs will be elected to single member constituencies and there would be a "top up" of another 100 or so seats distributed on the basis of the share of the votes achieved by the parties.

Space for debate

Proportional representation for Westminster elections is the long-cherished goal of the Liberal Democrats.

Liberal Democrat leader Paddy Ashdown has forged a co-operation pact with Prime Minister Tony Blair in an attempt to change the present system.

But Mr Cook appeared to suggest that while Mr Blair would welcome the report on Thursday, he would be in no rush to schedule a referendum.

Ashdown: Pact with Blair

He said: "I am quite sure that what Tony will want is to leave a space for the debate.

"I think that Tony on Thursday will probably want to allow the public to make up their own minds and to let that debate prgress before announcing his mind."

He accepted that the pressure of time would probably not allow reform to be carried out in this parliament, but said that a vote for change would be implemented by the next parliament.

"If Britain votes to change its electoral system I cannot believe that any substantial body of politicians is going to say 'we are going to ignore that choice of the people'", he said.