Tony Blair will be flying to meet his fellow EU leaders

At prime minister's questions, Mr Howard told MPs most voters wanted Mr Blair to say no to the constitution.

Mr Blair is due to join 24 fellow EU leaders on Thursday to try to clinch a deal over the future shape of the EU.

He accused the Tories of running towards the UK Independence Party "shouting 'me too'".

The Tories are seeking to keep the pressure on Mr Blair in a Commons debate.

Listening?

The success in the European elections of the UK Independence Party, which wants British withdrawal from the EU, has put more pressure on Mr Blair.

But he told MPs on Wednesday he believed Britain should stay at the centre of European decision making and not become marginalised.

The Tories say Blair has no mandate to sign a treaty

The government argues the constitution is needed to make the EU work effectively now it has 25 members, not 15 as before.

Ministers say the constitution will not fundamentally change Britain's relationship with the EU, although in a recent U-turn they have now promised a referendum on any treaty.

In the wake of the election results, the Tories are saying Mr Blair has no mandate to sign up to a constitution.

In their Commons clash, Mr Howard said: "The British people want him to say no. Which part of the world no does he not understand?"

Renegotiation?

The Tory leader said that even if the government secured all of its "red lines", Britain would give up more than 20 veto powers.

Mr Howard said he wanted to bring more powers back from Brussels and the constitution talks were a "huge opportunity" for doing so.

But the prime minister insisted it was impossible for Britain to renegotiate its terms of EU membership because that would need the agreement of every other country.

Mr Kennedy wants a concerted pro-European effort

"The true choice is either in or out," said Mr Blair. "I think it's about time he showed a bit of proper leadership and instead of running towards the UK Independence Party shouting 'me too' that he actually stood up to them and took on the argument."

He promised to defend British interests on issues like taxation, foreign policy and defence.

The Tory position or total withdrawal from the EU would be against Britain's interests, he argued.

Mood for a deal?

Earlier, Downing Street said the British stance in the negotiations would not surprise anybody - but stressed that the 24 other countries each had their own positions.

Mr Blair's official spokesman said he believed a deal was possible but added: "You do not know until you get there what the atmosphere is going to be and whether a deal will be on or not."

Ministers have said there is no way they will accept tax harmonisation and have also tried to quell fears about the Charter of Fundamental Rights.

There is some UK concern that concessions which ensured the European Court of Justice could not interfere with British employment laws have not been included in the draft text of the constitution.

No 10 says it wants to ensure that the charter does not impinge on British laws on strikes.

Campaign call

There are reports that France and Germany have complained they have already made enough concessions to Britain.

But Irish Prime Minister Bertie Ahern, who is chairing the talks, says he is confident of securing an agreement.

In Wednesday afternoon's Commons debate, Liberal Democrat foreign affairs spokesman Sir Menzies Campbell warned that "without leadership on Europe the voters will only hear one side of the story and that will inevitably be in negative terms".

The campaign for a "yes" vote should start now to have any chance of winning, he said.

"Being at the heart of Europe was one of the platforms on which this government was elected," said Sir Menzies.

"If a positive role is played this weekend and agreement is reached then that may go some way to meeting the government's pledge which in recent years has seemed to many of us to have become rather more of a plea."