LONDON -- A promised referendum on whether Britain should stay in the European Union on new terms, or quit the bloc, provoked fresh tensions within the British government on Wednesday and more blunt warnings from abroad.
In London, two cabinet ministers joined the introduction of a group campaigning to prevent a British exit from the bloc, with one senior member of the government appearing to raise some doubts that the referendum promised by Prime Minister David Cameron would actually go ahead.
"It is in our vital national interest that we avoid the fatal mistake that would be a 'no' vote if and when a referendum is held in the next few years," said Kenneth Clarke, a cabinet minister and one of the few pro-Europeans at the top of a Conservative Party where most lawmakers are critical of the European Union.
Drafts of Mr. Clarke's speech, released to the news media before the introduction of the group, had said, "If a referendum is held in the next few years," rather than "when."
Last week, Mr. Cameron promised a referendum within the next five years on whether to stay in the union on revised membership terms or to leave, but for his plan to go ahead, Mr. Cameron would first have to win the next election, due to be held in 2015, with a clear majority.
Mr. Clarke's draft speech added that Britain needed "to concentrate on what we are in favor of and not just what we are against."
Meanwhile, there were two warnings to Mr. Cameron about the risks of his bid to renegotiate membership terms before holding a popular vote. In The Times of London, the German foreign minister, Guido Westerwelle, said that there were no rights without duties and that there could be "no cherry-picking."
"We might be conjuring up forces that we can't control," he wrote, adding that it was easier to tear Europe apart than rebuild it. "We must not put at risk the common ground that we have achieved in more than half a century of European cooperation."
Speaking in Brussels, the Hungarian prime minister, Viktor Orban, said he was surprised that Mr. Cameron wanted to rewrite the European Union's treaty.
"I think it's very dangerous," he said. "It's a legitimate request, of course. I have nothing against it. But I would like to underline that this is the most dangerous thing I can imagine."
Mr. Orban explained that opening up treaty talks inevitably meant a range of other ideas "would come to the surface."
He also suggested that Mr. Cameron might be seeking to deflect attention from domestic challenges by raising the issue of renegotiating Britain's relationship with Europe.
"From a practical point of view, do not open the question of the modification of the treaty," Mr. Orban said. "Do our job, stabilize the economy, improve competitiveness, and be brave enough at home to do a proper job."
In Britain, Mr. Cameron's speech has been well received by his own lawmakers, providing a policy agenda around which his party can unite over a fractious issue. His lawmakers have been worried by the rise of the United Kingdom Independence Party, which wants Britain to quit the union.
With critics of the union in the ascendancy, Wednesday's introduction of the group, the Center for British Influence, was widely seen as the start of the unofficial campaign to mobilization support for a yes vote in a referendum.
Pro-European union pressure groups have enjoyed little success in recent years, the most recent being Britain in Europe, which was wound down last decade, having failed to blunt the advance of the Independence Party.
Danny Alexander, former head of communications for Britain in Europe and a Liberal Democrat who is now chief secretary to the Treasury, was the other cabinet minister who spoke at the pro-European event on Wednesday.
Mr. Alexander said big multinational companies "need to be reassured that we will continue to be the best bridgehead into the European market."
"We cannot afford to give the impression that we are going to disengage," he said.
Peter Mandelson, a former Labour cabinet minister and European commissioner, argued, "Those who want to destroy Britain's interests and influence in Europe have been allowed to get away with murder with the lies and false propaganda they have poured out about the European Union and what it represents for our country."
"This cannot go unchallenged any more," he added. "The pro-Europeans have bided their time. Now we must unbide our time."
Stephen Castle reported from London, and James Kanter from Brussels.
This article originally appeared in The New York Times.