The prime minister has branded Conservative policy on Europe as "fundamentally disingenuous".

Clashing with Michael Howard in the Commons on Wednesday, Tony Blair insisted he would stand up for Britain's interests at a European summit starting on Thursday.

Following last week's European elections, Howard asked if Blair would do "what the majority of the British people want him to do" and say no to the planned constitution.

Blair insisted he would "protect the national interest" but said Britain should remain at the centre of decision making.

Responding, Howard said the government's red lines were "red herrings".

"Everyone knows that this constitution transfers yet more power from Britain to Brussels," he added.

Warning that Britain would lose more than 20 vetoes, the Tory leader said he would "say no" to the treaty if he was in power.

Deal

But Blair said the Tory position "simply cannot be maintained".

"What we should be doing is securing a good deal for this country... that does not marginalise us in the European Union."

Howard said the constitution "provides this country with a huge opportunity" to take powers back from the EU.

But Blair said there were already provisions for "enhanced cooperation" so it would be "foolish" to reject it.

Branding Tory policy as "fundamentally disingenuous", the prime minister added: "The true choice is in or out."

And he said Howard should show "a bit of proper leadership" and take on the arguments of the UK Independence Party, rather than pander to them.

Debate continues

MPs continued to focus on the European constitution during a Commons debate later on Wednesday.

Foreign secretary Jack Straw said the planned treaty "gives us a chance to achieve reform of the way the European Union operates".

"We will have a single document which sets out what the European Union does and doesn't do," he said.

"It will make clearer than ever before that the European Union is a union of nations which freely choose to share certain powers in order to achieve objectives which they have in common and which only acts where its members have given it authority to do so and where it can add value."

Straw said there would be a greater role for national parliaments and a strengthened role for the Council of Ministers.

For the Conservatives, Michael Ancram told the foreign secretary the results of the European parliament elections had "invalidated his mandate" to agree the constitution.

And the shadow foreign secretary said agreement on the government's negotiating red lines "would be a hollow and deceptive victory".