In a major reversal of policy, British Prime Minister Tony Blair announced on Tuesday that he has agreed to a referendum on the proposed constitution for the European Union.

For months, the prime minister has been insisting that a referendum on the proposed E.U. constitution would simply not be necessary.

As Tony Blair acknowledged in the House of Commons, his political opponents and critical voices in the media have led him to the conclusion that a referendum is now necessary after all.

"It has been an unrelenting but, I have to accept partially at least, successful campaign to persuade Britain that Europe is a conspiracy aimed at us, rather than a partnership designed for us and others to pursue our national interests properly in a modern, interdependent world," he said.

Prime Minister Blair added that his critics have created many myths about the E.U. constitution and that his task is to convince the country that it really is in the best interests of all Britons to sign up to the document.

The British leader said that providing the constitution embodies key provisions such as a British veto on major issues of national interest, he then wants Parliament to debate the final wording, followed by a referendum.

However, he did not announce a date for the vote, which actually would come some months after work on the E.U. constitution is completed and then debated in Parliament.

The constitution is still being negotiated by E.U. members.

In a spirited exchange in Parliament, Mr. Blair said the time has come for Britain to decide once and for all whether to be at the center of European decision-making or to move to the margin and lose influence.

"Let the Euroskeptics whose true agenda we will expose make their case," he said. "Let those of us who believe in Britain in Europe, not because of Europe alone but because we believe in Britain and our national interests lying in Europe, let us make our case, too. Let the issue be put and let the battle be joined."

It will be an uphill struggle for Mr. Blair. The main opposition Conservative Party is strongly against the E.U. constitution and opinion polls show that the majority of Britons remain highly skeptical.

The timing of the referendum will also be crucial. Elections are expected to be called in 2005 and a referendum held either before or after that date will have important political implications.