The new EU constitution would introduce qualified majority voting on immigration and asylum, and create an EU president and foreign minister.
The EU constitution has prompted mixed responses
Tony Blair has said the UK will have a referendum on whether it should be adopted, although it is not clear when.
Supporters and critics set out their cases.
DEREK SCOTT, "NO" CAMPAIGN
This constitution gives more power to Brussels.
We disagree with some of our opponents who say it is a matter of being in or out of Europe, or that a "no" vote will mean that the country will be isolated.
We are focusing on the content of the constitution, which gives too much power to Brussels.
What we are seeing in Europe now, as more and more of the electorate see that the politicians they elect have less and less say over their own affairs, is the re-emergence of a very nasty form of nationalism - for example, in the Netherlands and Belgium.
It is perfectly clear to most people that the social and economic model entrenched in this constitution is not working and we need one that does.
If this constitution was accepted we would be facing a major political and economic crisis in two or three years.
I'm very gloomy for the prospects of Europe if this constitution is adopted.
LUCY POWELL, DIRECTOR BRITAIN IN EUROPE
The treaty creates a more flexible, open and accountable EU that exercises power only where and when member states say it can.
Importantly, Britain will keep its veto over tax, foreign policy and defence, so we protect our national sovereignty where we deem necessary, but can work more efficiently with other EU countries where desirable.
An enlarged union of 25 countries simply cannot operate effectively in the 21st Century from a rulebook designed for the original six members.
Britain is winning the argument in Europe and the ten new countries support our agenda for a reformed, economically liberal EU. Why jeopardise all that by voting no?
The treaty allows EU countries to work together to tackle global issues such as the environment, illegal immigration and cyber-crime.
It is not patriotic to campaign for a no vote that would marginalise Britain's standing in Europe and the world as we strive to do so.
Britain must have the self-confidence to Vote Yes to this treaty and remain a leading player at the heart of the EU.
NIGEL FARAGE, UKIP MEP
I want the "no" campaign to succeed not just in Britain, but also in France, the Netherlands, Poland and all the other countries.
I want this constitution to be put through the shredder and then I would like to see a big, open and honest debate about what the EU is for and whether we need these political structures or not.
I think a big "no" vote in several countries may lead to that.
We want to replace membership of the political union with a genuine free trade agreement. That is what people thought they were voting for 30 years ago.
We need to have a national debate about this. We have just had a general election where the three major parties ignored this completely.
For the first time opinion polls show the majority of Britons would rather have a simple free trade agreement rather than membership of a political union.
What we really need is Mr Blair to confirm that we are going to get a vote.
DAVID STEPHEN, DIRECTOR OF THE EUROPEAN MOVEMENT
This treaty is good for Europe and Europe is good for Britain.
The constitution brings together a lot of previous treaties and makes the enlargement to 25 member states easier to manage.
This treaty states what Europe is about in a clear and simple form.
It also makes the decision making process of the EU clearer and, for the first time, introduces a mandatory role for the European Parliament and national parliaments.
Therefore it is a step towards greater democracy in the EU's decision making.
Britain's involvement in Europe has been a success story.
Much of the "no" movement is about defeatism and little Englandism.