But his statement was clear -- Britain will hold a public vote, he said, on the draft European constitution.
"Provided the treaty [on the EU constitution] embodies the essential British positions, we shall agree to it as a government. Once agreed, either at the June [European] Council, which is our preference, or subsequently, Parliament should debate it in detail and decide upon it, then let the people have the final say [in a referendum]," Blair said.
The statement had been widely expected. It followed several days of media reports that the British prime minister was about to announce one of his biggest policy reversals.
For months, Blair had insisted Britain did not need to hold a vote on the planned constitution, as it would not infringe British sovereignty.
The government line is that the effects of the treaty will be benign -- that it will make the EU function better after it expands to 25 members on 1 May, and that Britain will retain control over its taxation, defense, and foreign policies.
Many opponents worry that the EU's draft constitution could undermine Britain's sovereignty by placing too much power in Brussels. And they're unhappy that the European Union will even have a constitution -- something they say only nation states should have.
If Denmark, Ireland, and Luxembourg could hold public votes on such an important issue, the argument went, then why not Britain?
It all left Blair's Labour Party in a vulnerable position as the only party opposed to a public vote.
Wyn Grant, a professor of politics at Warwick University, said there's another factor behind the government's U-turn. The backing of a group of popular newspapers -- seen as crucial to Blair's first electoral victory in 1997 -- could have evaporated if he had decided not to permit a referendum.
"New Labour really does need the support of that group of newspapers, which includes 'The Sun,' the largest-circulation daily newspaper, in the next election campaign. And it had been indicated that support would not be available if there had not been a referendum. I think another factor is that it really takes away from the [opposition] Conservatives' strongest ploys in relation to the European Parliament elections in June, but also indeed to the general election, the claim that the government was being elitist and arrogant in not permitting a referendum," Grant said.
So, there are some potential gains for Blair in announcing the referendum -- the continued support of an influential newspaper group, plus a possible boost ahead of upcoming European elections. And it shows he's listening to the people -- especially important after his deeply unpopular backing for the Iraq war.
But then there are the risks. A poll this week suggests that more than half say they would vote against the constitution. Only 16 percent say they would vote for it.
Blair has a reputation for being persuasive. Can he swing public opinion around this time?
Matthew Flinders is a lecturer in politics at Sheffield University. "It is risky,” Flinders said. “But I think actually, and this all goes to the issue of the time-tabling of when the referendum will be held, that Blair is not stupid. He will have taken a lot of advice about this, and if he didn't think that the British public were capable of being informed, brought up to date on the issues and making an informed judgment, he wouldn't have called it in the first place. So although at the moment the British public might not vote for it, I would suggest that quite a few people think that if done properly, the British public could vote positively for the constitution."
Flinders noted that Blair will be hoping to use the upcoming final negotiations on the draft constitution to his advantage.
"The Labour government has got to go to final negotiations over the European constitution and hammer out these final, very important sections about which policy areas remain those of the nation state. And if Tony Blair were to get what he wanted at the European level, it would make it much easier for him to take the European constitution and sell it to the British public, and he knows that," Flinders said.
The timing of the referendum is not yet clear, although it appears it will probably be held after the next general election, which is expected next spring.
That should give the government time to educate the public about its pros and cons.
But a more cynical commentator, Martin Kettle, writes in today's "The Guardian" daily of another possible reason Blair might have to push it back.
In order for the constitution to come into force, all 25 member states must ratify it. A long delay, Kettle writes, would "win time that will allow the Irish, the Danes, the Poles or even the French to kill the constitution first, before this week's desperate tactical pledge has to be redeemed."