Tony Blair with his briefcase at 10 Downing Street. (Reuters)

London, April 20 (Reuters): Tony Blair pledged today to give Britons the final say on a European Union constitution, a startling U-turn which will heap pressure on other EU big powers to follow suit and may define his future.

The Prime Minister told parliament that sceptical voters should have a referendum but only once parliament had “debated and decided” the planned charter.

“Then let the people have the final say,” Blair said. “Let the issue be put. Let the battle be joined.”

Until now he has insisted a constitution, likely to be agreed by EU leaders in June, would not fundamentally alter Britain’s ties with the bloc and so the public’s approval would not be sought. With polls showing the public could well reject it, the pro-European Blair is taking a big gamble.

His public trust ratings have plunged since waging war in Iraq, leaving the man who vowed to put Britain at the heart of Europe badly placed to swing mass opinion.

“The majority of the public are against... and it’s going to take a lot to persuade them to vote for it,” said Roger Mortimore of pollsters MORI.

But critics said Blair was acting out of domestic political expediency — neutralising a potentially damaging issue until after a general election expected in May 2005. Talks on the constitution collapsed at an EU summit last December. But a change of government in Spain — one of the major objectors — has helped revive negotiations and a deal is now likely in June.

Conservative leader Michael Howard, who opposes a constitution as a serious erosion of British sovereignty, taunted Blair who recently claimed he had “no reverse gear”.

“Today you can hear the gears grinding,” Howard jeered. “Who can trust him ever again?”