"Seeing Britain leave the European Union would be a tragedy," he added. "We must do everything for Britain and the British people ... to stay in the European Union. But of course not at just any condition, that wouldn't make sense."

Prime Minister David Cameron said on Monday Britain's vote to leave the European Unions must be respected and that the decision on when to trigger the formal "Article 50" exit process was a decision for "Britain alone".

"There can be no doubt about the result ... I am clear, and the cabinet agreed this morning, that the decision must be accepted," Cameron told parliament.

"The British government will not be triggering Article 50 at this stage ... this is our sovereign decision and it will be for Britain, and Britain alone, to take."