London and Brussels appear headed for stalemate going into a European Union summit on Tuesday discuss Britain's vote to leave.

With Europe's leaders divided over how to negotiate Brexit and Britain apparently reluctant to initiate formal talks on leaving, an EU source said lawyers had concluded that a member state could not be forced to launch the process.

But a senior EU official stressed that, by the same token, Brussels could refuse overtures for even informal talks before the exit process is officially initiated - a course that prominent Brexit leaders including Boris Johnson want to pursue.

"As long there is no notification, there will not be any negotiations," the official said. Brussels has given up hope that Britain could be bounced into triggering article 50 - the untested procedure that governs how a member state leaves the bloc - at the summit starting on Tuesday.

The official said the UK was in "a significant crisis" and it would be unrealistic to expect such a move. But he insisted: "We are ready to enter into this process quickly. We are ready to enter into this process as soon as possible."