In his first newspaper interview since his appointment, Mr Alexander told The Times yesterday that he hoped that other countries would ratify the treaty because it was right for Britain and for Europe.

But there is a big “but”. If the French say “no” on May 29, there is no Plan B, and it will be up to President Chirac and then all European heads of government to decide what to do. A “no” vote would clearly be a problem, the minister said.

Mr Alexander’s remarks summed up the dilemma facing the Government over the constitution, which led to this week’s conflicting messages that suggested first that there would be a referendum in Britain whatever happened in France, and second that there might not be.

There is a public message that Mr Alexander sought to put out yesterday. But privately many ministers say the opposite. Yesterday the Tories, sensing an attempt by the Government to escape a poll that they might not win, demanded a referendum at the earliest possible date: October 13.

Mr Alexander declined to repeat that there would be a vote here, come what may. “As long as there is a treaty to vote on, we will have a referendum,” he said. That was not what he had said the day before.

Behind the scenes ministers happily speculate that a “no” vote in France would get Tony Blair and Britain off a very big hook. M Chirac would have to accept defeat and say that he did not much like the treaty anyway. In which case it would be dead. Or he might suggest a negotiation to make it more to France’s taste. Reopening the treaty would be unacceptable to Britain and many others.

But ministers are having to work on the assumption that there will be a referendum and that they will have to win it.

And Mr Alexander was surprisingly upbeat about their chances. “My experience as a campaigner teaches me that you win campaigns when you win the arguments. I believe we have the arguments to win this referendum campaign. What emerged from the negotiations, a strengthened role for national governments and parliaments, a reduced commission and more effective decision-making, provides the framework for the kind of reformed Europe that Britain wants.”

So did he want a referendum? “Yes. If you read the Labour manifesto, pages 82 to 91 tell you why we believe the constitutional treaty makes sense for Britain and for our reforms in Europe.”

Britain takes over the presidency of the EU on July 1 and may have to pick up the pieces if there is a “no” vote in France, or in the Netherlands on June 1. Mr Alexander said that the Government would use the new Bill and the presidency to make the arguments about a reformed Europe that would help to win a referendum. If it happens it will almost certainly come next year.

In a speech in the City today, Mr Alexander will make economic reform the priority of Britain’s six months in charge. As one of the Chancellor’s closest allies he knows his mind better than most.

Mr Alexander will say that he wants to work with business across party lines to forge a “pro-reform, pro-European agenda”. He will promise to try to reduce the burden of European regulations on business. Britain will be pushing for action to assist the financial services sector and to strengthen US-EU economic co-operation.

Europe, he will say, must reach outwards to meet the global challenges, including the rise as economic powers of China and India. Again he was upbeat. “I believe we can win the argument about the nature of the EU we want to see.” At a recent meeting of trade ministers, the youthful-looking Mr Alexander said, he was one of the older ministers. There was an awareness among ministers and in the European Commission that economic reform was central to their work.

But on the constitution Mr Alexander maintained the parallel message. “We remain convinced that this treaty is right for France as well as right for Britain,” he said, adding: “If there was a ‘no’ vote in any member State then the response would be made by the government of the member state and then it would properly be brought to the European Council representing all 25 members.”

WHAT IF . . .

France and the Netherlands vote “no”?

Times Recommends

• Mandela’s tribe threatens President Zuma
• The French empire strikes back
• Wife of cheating governor exacts a political revenge