Mid-June is the deadline for sealing a deal on the constitution
Tony Blair is due to announce a referendum on the EU constitution, after a weekend of speculation and calls from the Tory opposition for him to "stop playing games".
BBC political correspondent Jonathan Beale explains what this sea change in government policy really means.
Why has Tony Blair suddenly granted this referendum on the EU constitution?
The prime minister has seen a growing campaign for that referendum, a campaign which involves not just Tory MPs, the Tory party, the Liberal Democrats, pro-Europeans but also many of his own MPs, Labour peers and he realises that it would be very difficult to get this through without a referendum.
For example, if it went to the Commons people would try to insert clauses that included a referendum.
He has decided that it would be more damaging not to have a referendum, the risks are greater and that is the reason why it looks likely he will call for a referendum.
We still do not know the crucial details about timing and how he is going to frame that debate.
Presumably Mr Blair is going to try to neutralise this issue in the forthcoming elections. Will he succeed?
He will have taken many by surprise if he is going to back a referendum, but there are still big questions about how he is going to frame that debate - whether he is going to make it into a much bigger argument about Britain's relationship with Europe and also the timing of when such a referendum should take place.
Could it take place before a general election? That's the question the Tories want answered... they believe it should, it should happen soon after the EU constitution is agreed.
It is expected to be agreed in June. I think Mr Blair will hope he has taken some of the wind out of the Tories' sails in time for the European elections to stop big votes against him but it is still a risky business.
He does not know whether this will win round sceptics, win round people who are very dubious about the government's relationship with Europe. So it is a high-risk strategy.
What does this new European constitution do? What is all the fuss about?
It depends who you listen to. If you listen to Tony Blair and his arguments so far - this is why he said there was no need for a referendum - it does not fundamentally change Britain's relationship with the European Union.
It is about simplifying the EU when it enlarges to 25 countries - that's going to happen in May.
But the Tories say this is a fundamental change, that there will be more majority voting, and Britain losing its veto in a number of key areas - but Tony Blair says that's wrong.
This debate has been going on and I think part of the problem for Tony Blair is that he has not convinced people yet that this is not a big change in the way the European Union and Britain are run.
Therefore he believes he has got to take this argument on, that he has got to win it and it seems and he has come round to the view that the easiest way to do this is to promise, some time in the future we do not know when, to have a referendum.