David Cameron has reportedly warned that Britain could leave the EU if Luxembourg ex-prime minister Jean-Claude Juncker becomes the next president of the European Commission. German news magazine Der Spiegel claims the Prime Minister issued the warning to German Chancellor Angela Merkel at the EU leaders summit in Brussels on Tuesday. Quoting sources close to those at the summit, the magazine said Cameron fears the appointment of continuity candidate Juncker would destabilise the UK Government to such an extent that it would bring forward an in-out EU referendum. Cameron reportedly added: "A figure from the Eighties cannot resolve the problems of the next five years."