But the prime minister retorted that once the constitution was approved by Parliament, "we'll have the debate in the country".

MPs from both sides heckled and jeered as the two leaders slugged out their opposing positions on the EU constitution.

Mr Howard asked the prime minister when his Cabinet reversed its policy to hold a poll on the EU constitution, but Mr Blair replied: "We announced our policy on the referendum yesterday."

Veto or renegotiate?

This prompted Mr Howard to mock that despite a "huge policy U-turn" and newspaper leaks "then you tell the Cabinet there's no time left for them to make a decision".

Mr Howard asked the PM to say whether he would veto the constitution or renegotiate the treaty if the 'no' vote wins the poll.

" Provided the treaty embodies the essential British positions, we shall agree to it "

Mr Blair said if that happened, the government would adopt a similar strategy to Ireland when it rejected the Nice Treaty for the first time, and sit down with other European countries to decide the way forward.

Later Mr Blair wrote to the Tory leader demanding to know if he would reject the constitutional treaty in its entirety or seek to amend it in discussion with the UK's European partners.

COUNTRIES HOLDING OR LIKELY TO HOLD A REFERENDUM:

Mr Howard said countries had constitutions while nation states made treaties with each other.

The government appeared ready to renegotiate the constitution in the event of a referendum 'no' vote, he said.

"In other words, if the British people did not vote the way you wanted, you would make them vote again until they did. You underestimate the British people."

Earlier, Trade Secretary Patricia Hewitt inflamed the Tories by claiming their "real argument" in campaigning for a 'no' vote in any poll was to withdraw from the EU.

STEPS TO A REFERENDUM