LONDON, April 20: British Prime Minister Tony Blair pledged on Tuesday to give Britons the final say on an EU constitution, a major political gamble that could threaten his future and perhaps even doom the charter.

His policy U-turn was greeted with concern in some European quarters - to enter into force, the constitution must be ratified by all 25 member states of the new enlarged European Union that comes into being on May 1.

With opinion polls showing Britons are highly wary of closer integration with the EU, one senior politician in the bloc accused Mr Blair of putting domestic politics before the interests of the Union and endangering the constitution.

Mr Blair told parliament his Labour government was ready to do battle in a referendum with the opposition Conservatives, who oppose an EU constitution as a threat to Britain's sovereignty.

Announcing a referendum would take place after parliament had "debated and decided", Mr Blair said: "Then let the people have the final say. Let the issue be put. Let the battle be joined."

Previously Mr Blair had said a constitution, likely to be sealed by EU leaders at a summit in June, would not fundamentally alter Britain's ties with the bloc and therefore public approval would not be needed.

Mr Blair's public trust ratings have plunged since he became US President George W. Bush's closest ally over Iraq, leaving the man who vowed to put Britain at the heart of Europe badly placed to swing mass opinion.

MAJORITY OPPOSED: "The majority of the public are against (an EU constitution)...and it's going to take a lot to persuade them to vote for it," said Roger Mortimore of pollsters MORI.