Boris Johnson: EU referendum before 2015 would be 'fantastic' However, it may be tricky to secure an arms-length relationship from Brussels. Francois Hollande, the French president, claimed on Friday that Britain cannot choose which laws it wants to follow from an "à la carte" menu and insisted the EU "is for life." Sounding a warning to Mr Cameron, the London Mayor said Britain has to be ready to walk away from the EU if the promised "fresh settlement" cannot be negotiated. "That is correct, absolutely correct [that Britain should be prepared to exit the EU]," he said. I don't think that [leaving the EU] is necessarily the end of the world. Don't forget that 15 years ago the entire CBI, British Industry, the City, everybody was prophesying that there would be gigantic mutant rats with two or three eyes swarming out of the gutters, the sewers, to gnaw the faces off the remaining British bankers because we didn't go into the euro. But my preferred option is for us to stay in there. I will stress [leaving] is not my preferred option. Sources close to the Prime Minister dismissed the idea that he would have trouble negotiating a new relationship with Europe. "David Cameron has been the toughest Prime Minister we've ever had on Europe," the source said. At this week's summit he was tough on them over issues like the single market and Syria. He has shown he can be tough all along. Mr Johnson is considered a potential Conservative leadership rival to Mr Cameron, even though the former MP has another three years left in his current job as mayor. He has repeatedly denied any ambitions to be prime minister and yesterday stressed again that he is not planning to return to the House of Commons in 2015. "I've got three and a half more years as Mayor of London and I'm going to stick to that," he said. Because I think we did a great Olympics but there's an awful lot more of that that needs to get delivered. We need to show that we can get value from that £9.3 billion. The Olympic investments are going to be transformatory of huge chunks of East London and I want to be there to make sure that we get it right.