In his biggest and most risky policy U-turn to date, the PM told parliament on Tuesday of his intention to hold the ballot after the next general election - expected in 2005.
His about-face follows a renewed commitment by EU leaders to finalise the constitution in June.

The ballot may also be in response to opposition Conservatives who plan to renegotiate any constitution if they win the next election.

"Capitulation!" was one headline reaction in the eurosceptic Daily Mail tabloid, which has mounted a campaign for a referendum.
The Guardian newspaper said "if it results in a No vote - as many on both sides expect - it could cost him the premiership" it said in a front page report.
It added that Blair's reputation would also be damaged in Europe, "where a veto from eurosceptic Britain would cripple the constitution" - which needs to be ratified by all EU states.

Media backing?

In an analysis for Tuesday's Independent daily, political editor Andrew Grice asked why the prime minister was making such a "spectacular U-turn and taking his biggest gamble?" 

Blair's Government promised EU
vote before it came to power

Grice pointed his finger at media baron Rupert Murdoch – owner of Britain's Sun and Times newspapers.

Quoting sources in Blair's cabinet of senior ministers, the political editor claimed Murdoch has made it clear he can not support the re-election of a Labour government unless it offers a referendum.
EU constitution

The proposed constitution would replace the stacks of treaties on which the EU is built, with a view to making the bloc function more effectively after its 1 May enlargement from 15 to 25 member states.
Eurosceptics, including Britain's opposition Conservatives, fear the constitution will undermine Britain's national sovereignty.

But calls for a referendum have also been growing within Blair's own Labour party.
Cabinet heavyweights, including Chancellor of the Exchequer Gordon Brown and Foreign Secretary Jack Straw, have reportedly swung in favour of a constitutional vote.