Tony Blair last night appeared set for a dramatic U-turn by calling a referendum on the European Union constitution.

The Prime Minister has always insisted voters will not have a say on the controversial treaty, due to be completed in June.

But Mr Blair will reportedly seek Cabinet backing for a poll, at the weekly meeting on Thursday.

His deputy, John Prescott, yesterday confirmed the PM was listening to the argument for a referendum.

"We do consult the people where we think it is justified and necessary," Mr Prescott said.

Mr Blair fuelled speculation that he will change his stance when he refused to rule out a vote in an interview broadcast on Saturday.

"Our policy has not changed and if there is any question of it changing we will tell you," he said.

A senior Cabinet minister then told The Sunday Telegraph: "Tony has crossed the line. It's now really a question of timing."

Tory leader Michael Howard said: "If the Government really is planning a U-turn we welcome that, we have persistently called for a referendum.

"However, the Government must not play games with the question or the timing."

As well as the Conservatives who want to see the constitution rejected, Mr Blair faces pressure on the issue from Liberal Democrats, who back it, and some of his own backbench Labour MPs.

LibDem leader Charles Kennedy called on him to come clean on his plans for a referendum.

"I have long since welcomed that opportunity, and the Liberal Democrats have consistently made that case," he said.

"However, the cause of such necessary openness is not helped by off-the-record briefings to newspapers and a series of nods and winks to selected individuals."

Conservatives are making a referendum a central issue of their campaign for the European elections on June 10, when Labour is expected to take a hammering.

There have been repeated suggestions that Mr Blair is rethinking his opposition to a poll in the face of that.

The PM had argued the vote was not needed because the treaty would not alter Britain's relationship with the EU. But on Saturday he hinted Tory threats to renegotiate the deal if elected in the European elections in June could trigger a poll.

"The thing that has changed recently is a radical change in the Conservative position, which is no longer to say they don't support this constitutional treaty, but they would re-negotiate it if it was passed," he said.

Cabinet heavyweights, including Chancellor Gordon Brown, had swung in favour of consulting voters, according to recent reports.

A poll could now happen before the next General Election, expected in about a year, a colleague was quoted as saying yesterday.

However, Mr Brown is said to be cautious and concerned that an early poll could complicate a vote on the euro if it is decided the time is right to join after the next election.

The constitution has been drawn up to streamline decision making when the EU expands in May. However, critics say it is far more than the tidying up exercise ministers have suggested and will strengthen the powers of the EU.

Sir Menzies Campbell, Liberal Democrat foreign affairs spokesman, said: "If these reports are true they will represent an enormous volte-fare on the part of the Government."