Britain must begin exit talks "as soon as possible", European Union leaders said as they battled to shore up the bloc and prevent a contagion of copy-cat referendums pulling it apart. The country is on course for a high-stakes stand-off with Brussels after leaders in a joint statement said "orderly" talks must begin without delay, "however painful that process may be". Britain alone can trigger Article 50, the EU's exit clause, but the EU is now prepared to exert intense pressure to kickstart talks. Boris Johnson, the former mayor of London, yesterday claimed there is "no need for haste" in leaving the bloc. Mr Cameron said it would be up to the new Prime Minister - expected in office by October - to decide when to trigger the clause. "We are determined to keep our unity as twenty seven," said Donald Tusk, the Polish president of the European Council. "Uncertainty is the opposite of what we need," said Martin Schulz , the President of the European Parliament said, saying he could not accept "a whole continent is taken hostage because of an internal fight in the Tory party". Activating Article 50 begins a two-year negotiation period, during which all current rules apply. While that provides stability, it also bars Britain from agreeing simultaneous trade deals. Angela Merkel, the German Chancellor, appealed for calm yesterday, saying that leaders must avoid "quick and simple conclusions" and must instead "calmly and prudently analyse and evaluate the situation." She will host talks with Francois Hollande, the French president, Matteo Renzi, the Italian Prime minister, and Donald Tusk, the European Council president, on Monday. Mr Cameron will join his 27 former counterparts at a summit on Tuesday, but will be excluded from the session on Wednesday. Mr Cameron will "explain the situation" to his counterparts over dinner, followed by an "exchange of views" with his counterparts, Mr Tusk said in a letter sent last night. Mr Hollande said it was a "grave test for Europe". He warned: "It always takes less time to undo than to do, to destroy than to build." Xavier Bertrand, the president of the Calais region, demanded that the Le Touquet agreement that allows British border checks to be conducted on French soil to be abandoned - a move that could result in illegal migrants reaching Britain and setting up camp. "The English wanted to regain their freedom; they must take back their border," he said. Mr Schulz said Britons must face "consequences" in order to avoid a "chain reaction" of referendums.