Ahead of a key summit later this week, Gordon Brown and David Cameron have traded blows on the new European treaty.

National heads of government and state meet in Lisbon on Thursday and Friday to sign off plans for a replacement to the constitution.

But ministers remain under pressure on their decision not to hold a referendum ahead of ratification of the new document.

Speaking in the Commons, the Conservative leader insisted that the "overwhelming majority of people in this country want a referendum on the European constitution".

"Will he tell us, why won't he grant a referendum on that constitution?" Cameron asked.

Brown argued that there had been no referendum when the then Conservative government passed the Maastricht treaty.

"In 1992 every member of that shadow cabinet refused a referendum on that far more significant treaty," he said.

And he insisted that this treaty "is not a constitutional treaty, it is an amending treaty".

A series of changes had been "brought about in the last few months" to ensure that Britain's interests are protected, he added.

But Cameron noted that Labour MP Gisela Stuart had backed the case for a referendum, saying it was "a matter of trust and integrity".

Responding, Brown said: "We will do what is right in the interests of the British people."

And he quoted former Conservative chancellor Ken Clarke a saying a referendum would be "frankly absurd".

The Conservative leader insisted that the government should keep its pledge to hold a referendum.

"People will look at him and just think, here is a man who breaks his promises," said Cameron.

And he warned Brown: "If he breaks his promise on this, no one will trust him on anything else."

The prime minister told MPs that the new document would be "an amending treaty" without fundamental change to existing EU structures and rules.

The UK will continue to decide on key issues such as justice, foreign affairs, and social security, he said.

"We will at all times stand up for the British national interest," Brown added.