David Cameron's European Union referendum promise is a "very risky strategy" that will cause four years of uncertainty, the man challenging Angela Merkel to become the next German chancellor has warned.
Peer Steinbrueck, the candidate for the centre-left Social Democrats in this September's elections, said that treaty change would set off a string of referendums - with unpredictable consequences - across the continent.
His comments came as French president Francois Hollande insisted there could be no "a la carte" Europe - a stance backed by Mr Steinbrueck after talks with Labour leader Ed Miliband.
Speaking alongside Mr Miliband in Westminster, Mr Steinbrueck said he wanted Britain to stay in the EU, adding: "I think after this speech, the promises Mr Cameron made, this is a very risky strategy."
He said it would cause "a lot of uncertainties" between now and 2017. He said it was "necessary to improve some procedures" in European institutions but not through treaty change.
"An amendment of these European treaties will cost a long, long time and in my eyes it's for sure that afterwards many countries will have to organise referendums," Mr Steinbrueck said. "Nobody knows what is the outcome of these referendums."
Speaking to the European Parliament earlier, Mr Hollande called for a "multi-faceted Europe which would be neither a two-speed Europe nor an a la carte Europe".
Asked about the French president's position, Mr Steinbrueck said: "It is not necessary to follow Mr Cameron that we make all these specific deals with the UK, with certain exceptions, so I agree with president Hollande that it couldn't be and can't be a Europe a la carte."
Mr Miliband said that exiting the EU was "the wrong thing to do for Britain" and that the focus should be on an economics agenda of growth, jobs and living standards which were "at the core of making Europe work for Britain".