New resistance to Cameron's referendum plan on the EU David Cameron ... warnings from abroad. 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 the 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, one of the few pro-Europeans at the top of the Conservative Party. Drafts of Mr Clarke's speech, released 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 EU on revised membership terms or to leave. 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 favour 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, 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. We must not put at risk the common ground that we have achieved in more than half a century of European co-operation. Speaking in Brussels, the Hungarian Prime Minister, Viktor Orban, said he was surprised that Mr Cameron wanted to rewrite the EU'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 suggested that Mr Cameron might be seeking to deflect attention from domestic challenges.. "From a practical point of view, do not open the question of the modification of the treaty," Mr Orban said. Do our job, stabilise 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 politicians, providing a policy agenda around which his party can unite over a fractious issue. His politicians have been worried by the rise of the United Kingdom Independence Party, which wants Britain to leave the EU. With critics of the union in the ascendancy, Wednesday's introduction of the group, the Centre for British Influence, was widely seen as the start of the unofficial campaign to mobilise 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.