The anti-euro No Campaign is to be wound up, following Gordon Brown's decision to rule out a fresh assessment of the five economic tests for entry.
Campaigners said there was "no chance" of a euro referendum before 2008.
The move comes as the campaign for a referendum on the European constitution prepares to shift into top gear.
French president Jacques Chirac has reignited the controversial issue by saying he hoped a deal could be reached at an EU summit in June.
Earlier talks fell apart in December after Spain and Poland united to block a deal on voting rights.
But the election of a pro-European government in Spain had revived hopes of an agreement at the EU summit on 17 and 18 June - a week after the European elections.
Irish premiere Bertie Ahern, whose country currently holds the EU presidency, has said he will restart treaty negotiations, following talks at the weekend with Mr Chirac.
But the move could provide a headache for Tony Blair, who is facing calls for a referendum from opposition MPs and peers and sections of the press.
Campaign group Vote 2004 said Mr Chirac's comments over the weekend had effectively fired the starting pistol on the campaign for a referendum.
"It is game on as far we are concerned. The phoney war is over," he told BBC News Online.
The government is reportedly keen to get the constitution issue out of the way before the next general election.
But if enough peers back an amendment calling for a referendum, Mr Blair may be forced to use the Parliament Act to get the legislation on the statute book, Mr McManus argued.
"If that happens, they will be using an ancient act quite improperly, on a measure which was not in their manifesto, to deny the people of this country the opportunity to decide on their constitution.
"They should ask themselves if they really want to go down that route. It is pretty high stakes."
Meanwhile, the No Campaign, which was formed in September 2000 from Business for Sterling and New Europe and shares many of its supporters with Vote 2004, has said it will close its office within the next month, with the loss of three jobs.
A steering group will continue to monitor the euro debate and will be ready to "gear up again if the government goes for a referendum regardless of the economics".
No Campaign chairman Nigel Smith said: "It is our judgement there is no chance of that referendum being called in the near-future and it is right to end active campaigning.
"We have built the broadest coalition of people opposed to joining the euro - including representatives of six political parties. In a way we have been a victim of our own success".
The pro-euro Britain in Europe group is expected to carry on with its campaign, and recently appointed a new director, Lucy Powell.
But in a rare speech on the economy on Monday, Tony Blair appeared played down hopes of early euro entry, saying Britain would only join if the existing five economic tests were passed and doing so would enhance stability.