Former German foreign minister Joschka Fischer has warned British PM David Cameron not to be too sure of German support in his bid to reform the EU.
It comes a week after German Chancellor Angela Merkel pledged to try to find a solution to the UK's drive for reform.
Mr Fischer, an architect of European foreign policy, told the BBC that the UK was not a priority for Mrs Merkel.
Mr Cameron is starting renegotiation of the terms of Britain's EU membership ahead of a referendum.
But Mr Fischer said his advice to David Cameron was: "Don't lose yourself in wishful thinking.
"Angela Merkel will do nothing which will endanger the basic principles of the common market, of the EU," he told BBC Europe editor Katya Adler.
"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."
Mr Fischer said it would be an "illusion" to think the UK would get special treatment because it is a major contributor to the EU budget.
He added: "What will the UK be without the EU? Go to Washington, ask them. The answer is very clear, very negative."
Last week, Mr Cameron made a whistle-stop tour of Europe trying to gather support for changes he wants before holding the UK's EU membership referendum.
After talks with the UK prime minister in Berlin, Mrs Merkel said she did not rule out future treaty changes in Europe.
"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," she said.
Mrs Merkel said her "clear-cut" view was that the UK should stay in the EU.
Mr Cameron has not set out in full detail the reforms he wants but analysts say they will include tougher rules to prevent migrants claiming benefits.
He also wants safeguards to protect the City of London in the event of closer eurozone integration and an exemption for Britain from the EU drive for "ever closer union".
The UK's in/out referendum will take place before the end of 2017.