Germany and France have warned UK Prime Minister David Cameron that Britain cannot pick and choose EU membership terms after he pledged a referendum.

Mr Cameron said a poll would be held if the Conservatives were returned to power at the next general election, which is expected in 2015.

Voters would be asked to choose between renegotiated membership or exiting.

Germany said the UK could not “cherry-pick” while France said “a la carte” membership was not on the table.

However, in an apparent concession to Mr Cameron’s concerns, German Chancellor Angela Merkel called for a “fair compromise” between the wishes of Britain and other EU states.

BBC Europe editor Gavin Hewitt says that Mrs Merkel is, as usual, being cautious. She wants to explore ways of keeping Britain in Europe but ultimately she is committed to more Europe not less Europe.

In Washington, the White House welcomed Mr Cameron’s “call to remain in the EU”, saying it believed that the UK was stronger as a result of its EU membership.

UK’s ‘destiny’

In his long-awaited speech, Mr Cameron said the referendum would be a decision on the UK’s “destiny” and, if he secured a new relationship he was happy with, he would campaign “heart and soul” to stay within the EU.

But he did not spell out what powers he would like to see the UK take back as part of a new settlement, or what would happen if the negotiations did not go his way.

Mr Cameron argued that “disillusionment” with the EU was “at an all-time high” and “simply asking the British people to carry on accepting a European settlement over which they have had little choice” was likely to accelerate calls for the UK to leave.

“That is why I am in favour of a referendum,” he said. “I believe in confronting this issue – shaping it, leading the debate. Not simply hoping a difficult situation will go away.”

Mr Cameron said he believed Britain’s national interest was “best served in a flexible, adaptable and open European Union and that such a European Union is best with Britain in it”.

The many Eurosceptics in Mr Cameron’s Conservative Party will be pleased he is offering the referendum although some will regard the timescale as tardy, BBC political correspondent Iain Watson reports.

Opinion polls

Reaction to Mr Cameron’s long-awaited speech in London came swiftly.

French Foreign Minister Laurent Fabius said a future for the UK outside the EU could be “dangerous”.

“We want the British to be able to bring all their positive characteristics to Europe… but you can’t do Europe a la carte,” he told French radio.

“Imagine Europe is a football club and you join, but once you’re in it you can’t say, ‘Let’s play rugby’.”

French President Francois Hollande was clear about his desire to see the UK remain an EU member, his spokeswoman Najat Vallaud-Belkacem said.

“[But] being a member of the European Union involves obligations,” she added.

An online poll in France’s centre-right newspaper Le Figaro suggested many French people would be happy to see Britain leave.

With more than 15,500 votes cast, 70% favoured the UK leaving over 30% who disagreed.

Mr Cameron’s name was trending among French users of Twitter as of Wednesday afternoon.

While it appeared not to rank high among German tweeters, a phone poll by German broadcaster n-tv on whether Britain should leave the EU found 80% of viewers in favour of exit, to 20% who disagreed.

‘More integration’

German Foreign Minister Guido Westerwelle called for the UK to “remain an active and constructive” part of the EU.

He said: “We need a new commitment to the principle of subsidiarity.

Not all and everything must be decided in Brussels and by Brussels.”

However, the answer to the Union’s economic troubles, Mr Westerwelle argued, was “more integration”, not less.

“But cherry-picking is not an option,” he added.

His opinion was echoed by Austrian Foreign Minister Michael Spindelegger, who said the EU could only succeed economically “if you pull together in the same direction”.

Danish Prime Minister Helle Thorning-Schmidt was unimpressed with Mr Cameron’s speech, arguing that “a flexible Europe” already existed and Denmark – with its own EU opt-outs – was a good example.

Europe should not be a “help-yourself table”, Politiken newspaper quoted her as saying.

The response from most European capitals is clear, the BBC’s Chris Morris reports from Brussels – we don’t want Britain to leave, but when you join a club, you have to abide by the rules.

But when David Cameron speaks of an EU as something done to people rather than acting on their behalf, that will certainly strike a chord with those who have protested in Athens or Madrid against policies over which they feel they have little control, he adds.

If the British referendum does go ahead, it will be held by the end of 2017 at the latest.

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