David Cameron has been warned by a former German foreign minister not to overestimate the support he will receive from Berlin in his push to renegotiate Britain's relationship with the European Union.
The Prime Minister was encouraged last week by Chancellor Angela Merkel's stated desire to play a "constructive" role as he seeks sufficient reforms to persuade UK voters to back continued membership in a referendum.
He has promised to put an in/out choice to the electorate by 2017.
But Joschka Fischer warned that the issue was only a secondary priority for Mrs Merkel - whose support will be crucial to any renegotiation - and that she would not yield over any "basic principles" of the EU.
"Don't lose yourself in wishful thinking," he warned Mr Cameron in a BBC interview.
"Angela Merkel will do nothing which will endanger the basic principles of the common market, of the EU," he told the BBC.
"And she has a much bigger problem to address - how to find a compromise in the currency union with Greece. That's her priority number one now."
After talks with Mr Cameron as part of his opening gambit whistlestop tour of potential allies last week, Mrs Merkel did not rule out EU treaty change and said Germany has "clear cut hope" that Britain will remain a member.
Mrs Merkel said: "Of course we have the desire to work very closely together. We would like to be a part of the process that is going on in Great Britain at the moment and we would like to be a constructive partner in this process.
"I have also said wherever there is a desire there's also a way and this should be our guiding principle here as well."
Mr Fischer suggested that the UK could not afford politically to quit the EU.
"What will the UK be without the EU? Go to Washington, ask them. The answer is very clear, very negative," he said.