British attempts to claw back powers from Brussels would put the future of the single market at risk, the president of the European Council has claimed.
In a stark warning Herman Van Rompuy cautioned against any attempt to "undermine" the union and claimed cherry-picking policies would see the EU "unravel".
A British exit would "see a friend walk off into the desert", he added.
Prime Minister David Cameron is due to make a keynote speech on Europe early next year and has said there would be "opportunities for changing our relationship" with Brussels.
In an interview with The Guardian, Mr Van Rompuy said: "If every member state were able to cherry-pick those parts of existing policies that they most like, and opt out of those that they least like, the union in general, and the single market in particular, would soon unravel.
"All member states can, and do, have particular requests and needs that are always taken into consideration as part of our deliberations. I do not expect any member state to seek to undermine the fundamentals of our co-operative system in Europe."
He added: "Britain's contribution is greater, I think, than it sometimes realises itself. It has been crucial in building the EU's centrepiece, the single European market, now the largest market in the world, and the common rules for the common market that are necessary for it to function.
"British expertise in the fields of foreign policy, finance and trade shape the EU's policies in these fields. It has led the way on climate change and development aid. It has offered us the English language, now in practice the lingua franca of Europe."
Wolfgang Schauble, German finance minister, told The Guardian: "Without the EU as an amplifier, Britain's influence in the world would be lessened. No European country alone can make its voice heard in today's globalised world."