By Mark Mardell
BBC Europe Editor
The Italian prime minister has told the BBC that reviving the European constitution is fundamental to Europe's future survival.
Romano Prodi, who was also president of the European Commission from 1999 to 2004, said that Britain had to decide whether to stay in or not.
A divided Europe will not survive, Mr Prodi says
Mr Prodi, speaking ahead of celebrations to mark the 50th anniversary of the signing of the Treaty of Rome, which established the Common Market, said he was for the constitution.
"We have to have it before the European parliament elections of 2009 because we are 27 countries and we need the rules," he said.
"Like it or not, if we have a united Europe we survive. If not, we are dead. I am absolutely convinced that process is irreversible.
"We need time. We needed 45 years to have the euro; I don't know how long before we have the constitution but we need it," he added.
The European constitution was voted down by the people of France and the Netherlands two years ago, but has been endorsed by 18 countries.
The Germans, who currently hold the EU presidency, are determined to bring it back in one form or another.
Although the Declaration of Berlin, to be signed this weekend, will carefully not mention the word "constitution", it is likely to set the date of 2009 for "renewing the basis on which the EU is built".
Angela Merkel also backs the idea of a European constitution
The Germans hope that by the time of the next big meeting of EU prime ministers and presidents in June, they will have a plan on how to revive some parts of the constitution, although the word is likely to be dropped.
The rule changes would be contained in a new treaty.
Mr Prodi said: "Step by step we shall go back in order to have a common basic paper: probably it will be less complete, and maybe in some people's opinion less cumbersome, but it will include all the basic principles, not the specific rules of behaviour."
Only one of the French presidential hopefuls, Segolene Royal, has said she would hold a referendum on a new treaty.
The Dutch have not said what they would do.
There would be pressure in Demark for a popular vote.
The British government is nervous of repeating its promise to hold a referendum which it fears could be lost.
Ministers hope that any treaty changes would be seen as too minor to trigger such a vote, and much would hang on how the conservatives decide to play the issue.
"From the constitutional point of view, I think that only Ireland needs it, but any country will choose," Mr Prodi said, adding: "If the UK wants a referendum why not? They will decide whether to stay in or not."
When I asked him what he meant by this, he said: "There is a basic rule that is fundamental: with our rules, if you say no, no, no then you are in a moment in which you have to make a decision. It will be the start up of a new discussion".