IN May last year, Tony Blair could "see no case for having a referendum" on the proposed EU constitution; two months later he said a referendum would only be justified if it changed the basic relationship between Britain and the EU, adding: "I don't believe this does that", and just in case there was any remaining doubt, in October he was emphatic, saying: "There will not be a referendum".

Fast forward to yesterday, and Mr Blair stands up in the House of Commons to declare it is time to "let the people have the final say". There will be a referendum on the constitution after all. He's not yet saying when, but at some point there will be the first nationwide referendum since the 1975 poll on whether Britain should stay in the Common Market.

So what has caused the biggest U-turn of the New Labour government? What is the new constitution all about? And what will happen if Blair loses?

Q What's the idea of an EU constitution?

A In 11 days time, ten new countries will join the EU, bringing the total number of members to 25. But the EU is still running on rules designed for the six founding members, modified by subsequent treaties. Supporters of the constitution argue that the EU needs to be streamlined so it can work efficiently after the expansion, and so propose replacing the treaties with a single document, saying what the EU can and cannot do. It is unlikely to come into effect before 2006, even if it is ratified by all member states.

Q What does the draft constitution say?

A It ranges from setting out a vision for Europe's future, to details about how member states can leave. Proposals including creating an EU foreign minister and permanent president; giving the European Parliament more power; setting out procedures for suspending a member state, and making the EU's Charter of Fundamental Rights part of the constitution.

The main sticking point between the member states is about the voting rights in the EU Council of Ministers, with the larger countries wanting a system based on population size, but the smaller countries fearing this would leave them at the mercy of the big four: France, Germany, Italy and the UK.

There are also disagreements over the number of commissioners each country should have; whether member states should retain vetoes in foreign policy, defence and tax; a mutual defence clause which requires member states to come to the aid of another EU country, and whether the European Parliament should have the final say over the EU budget.

QWhat difference will it make to our lives?

AProbably not very much, although the president and foreign minister could become high profile figures. Opponents argue that the constitution will see member states hand over too much power to the EU, and is the first step towards creating a United States of Europe.

Q Why did Tony Blair initially oppose a referendum?

AHe said the constitution did not fundamentally change Britain's relationship with the EU, so there was no need to hold a nationwide vote. Peter Hain, the Leader of the Commons, described it as merely a "tidying-up exercise".

Q So why has Blair changed his mind?

A The prime minister has come under pressure from the Tories and Liberal Democrats, as well as parts of the media, to hold a referendum. The prime minister has remained firm in the face of these demands, but what seems to have changed is that several Cabinet heavyweights have come out in favour of a poll. Gordon Brown, Jack Straw and David Blunkett are all said to have urged the PM to perform a U-turn, partly to take the wind out of the Tories' sails.

The Conservatives were looking to base their campaign in the European elections in June on putting the constitution to a referendum, so by agreeing to one, Blair has stolen what was looking like becoming fruitful ground for Michael Howard.

Added to this, the Government fears the legislation to approve the constitution could have a difficult time going through Parliament, making sure the controversy is kept alive. A referendum will both dampen parliamentary opposition, and neutralise the constitution as an issue in the next general election.

Eight of the 25 countries in the enlarged EU have already offered their people a referendum, with others thinking of following suit, making arguments that it is not important enough sound hollow. Denmark, Ireland, the Netherlands, Luxembourg, Portugal, Spain, the Czech Republic and Poland have all said they will hold a referendum on the constitution, while only Italy and Germany have so far ruled one out. Blair's new strategy has the added advantage that if any of these referendums goes against, it will be back to the drawing board in any case, so he can have promised a vote without having to actually hold one.

All in all, the fact Blair has embarked on such an embarrassing U-turn shows how much trouble he believes is in over the constitution.

Q When will the referendum take place?

A Michael Howard wants it to happen as soon as possible, but Blair wants Parliament to discuss it first, which means it is unlikely to be before the next election.

This will limit the risk to his own leadership, which would be looking precarious if the referendum is lost.

This also allows time for Blair to try and turn public opinion around, giving him the option of winning both the next election and the referendum, before standing aside while he's still on top.

Q So does Blair think the referendum can be won?

A All the polls are saying there is a clear majority against the constitution, but the only precedent has good news for the Prime Minister. At the start of the 1975 referendum campaign the polls showed 55 per cent against staying in the Common Market, with just 45 per cent in favour. But when it came to the vote, 67 per cent were in favour and just 33 per cent against, a swing of 22 per cent.

The difference then was that the leadership of both major parties favoured staying in, and it was largely marginal figures including Tony Benn and Enoch Powell who campaigned for withdrawal. Even so, Blair must feel that a major campaign could shift public opinion. There has been speculation he could try and turn it into a vote on Britain's future in the EU, although this line could come a cropper if other member states don't make the same link.

For Blair, victory in the poll represents a chance to end the rows over Europe which have dogged British politics for the last 30 years.

QWhat happens if he loses?

AIf the referendum takes place before the general election, it's difficult to see him staying on if the vote goes against him. Pressure to go from within his own party would be hard to resist, and it would have become clear that any remaining trust in his leadership had evaporated. But if the poll comes after the general election, he may see this as the ideal time to step aside. This would be easier if the referendum had been won, but if it had been lost he and the party may decide it is time for someone else to take the reins.

Q What about the referendum on the euro?

A This is still on the agenda, when Chancellor Gordon Brown judges the time is right. There have been suggestions both polls could be held on the same day, but Brown is likely to oppose that on the grounds it would blur the issue. More likely is that a referendum on the constitution is followed by one on the euro.