It is essentially a rule book setting out what the EU can and cannot do. It sets down in writing the EU's values and political objectives.
The constitution is designed to streamline decision-making in the enlarged EU of 25 states. For example, it lowers the size of the majority needed for most decisions in the European Council, and cuts the number of areas where a unanimous vote is required. But it also opens the way to deeper EU integration and greater centralisation of decision-making. Opponents argue it will effectively turn the Union into a country in all but name, with its own flag, parliament, civil service, anthem, supreme court and president.
Tony Blair will lead the campaign for a yes vote, with the rest of the Labour front bench playing a prominent role. Mr Blair is a relatively recent convert to the idea of a constitution, previously preferring to talk about a "charter of competencies". He now argues the constitution is in Britain's interest and claims to have secured a good deal in treaty negotiations, protecting the UK's vetoes on economic policy, defence and foreign affairs. Mr Blair is likely to share a platform with Liberal Democrat leader Charles Kennedy and his front bench team - all passionate advocates of closer EU integration. Britain in Europe, the pro-EU lobby group backed by some of Britain's biggest companies including BT, Unilever and Ford, is also likely to be involved. Newspapers including The Daily Mirror, The Independent and The Guardian, are also expected to back a yes vote.
The Conservatives want to see limits set to the EU's power, and are not in favour of a constitution. Instead they want a new treaty that would shift some powers back to nation states. The UK Independence Party, which wants British withdrawal from the EU, the Green Party and the anti-war Respect coalition, also back a no vote. Up to 70 backbench Labour MPs, under the banner Labour against a Super State, are also likely join the no campaign. Vote No, a group backed by business leaders including Wetherspoons pub boss Tim Martin, Dixons chairman Lord Kalms and Sir Anthony Bamford, of engineering group JCB, will also be a leading voice. The Sun, Daily Mail and Daily Telegraph are among the newspapers expected to throw their weight behind a no vote.
Ten other EU countries have so far said they will hold referendums. Spain became the first country to approve the constitution by referendum on 20 February. The upper house of parliament has yet to complete ratification. The constitution was backed by 77% of voters, with 17% against in the referendum, which was not legally binding. With all of Spain's main political parties in favour, ratification is all but guaranteed.
Tony Blair has said he will hold a vote during 2006.
The constitution can only be adopted by the EU if it is ratified by every member country. Tony Blair has said he would respect a no vote - and he has ruled out holding repeated referendums until he gets the answer he wants. But he has also argued that a no vote would amount to British withdrawal from the EU. The Tories say there is nothing to prevent the EU continuing without a constitution. The EU's foreign policy chief, Javier Solana, said a UK no vote would not exclude Britain from the "European family".