Blair will now let the people vote.
A referendum on a constitution would be the first major gauge of public opinion on the EU since the 1975 referendum on whether Britain should stay in the Common Market.
Outlining his radical U-turn, Mr Blair told MPs it was time to resolve “once and for all” whether Britain wants to be at the centre of Europe or not.
In a Commons statement, he said Parliament should debate the constitution question “in detail and decide upon it” and “then let the people have the final say”.
“The question will be on the treaty, but the implications go far wider,” he said.