Prime Minister David Cameron bid goodbye to his European Union counterparts in a "sad" Tuesday summit in Brussels, while leaders' hands were tied on follow-up action after Britain's momentous decision to leave the bloc.

Britons made the choice in a Thursday referendum that triggered political mayhem in London, caused global market panic and reverberated across the EU. The vote should separate the EU from one of its most influential members.

"Our partners in the EU are genuinely sad that we are planning to leave this organization," Cameron said, adding that he had constructive, positive and calm discussions and was leaving convinced that Britain and Europe should seek "the closest possible relations."

Going into the summit, the British leader faced criticism for leaving to his successor - to be elected in September - the responsibility of starting the exit process by activating Article 50 of the EU treaty, rather than initiating divorce proceedings immediately.

EU Council President Donald Tusk said there was some "understanding that some time is needed" before London can make a move, but said Cameron was asked to "specify as soon as possible" when this would happen. "This was a very clear message," he stressed.

European Commission President Jean-Claude Juncker showed some sympathy for Cameron, who had campaigned for Britain's continued EU membership and announced his resignation after his arguments were rejected by voters.

Juncker said he was amazed that the winning Leave camp was so unprepared for their victory. He described them as "totally unable to tell us what they want."

EU foreign policy chief Federica Mogherini questioned whether Britain really wanted out. "The British people were quite clear on this, but we are receiving contradictory messages from ... a rather confused political scene in London," she said.

But German Chancellor Angela Merkel warned there was no way back. "I see no way to reverse that," she said of the referendum outcome. "This is not the hour for wishful thinking," she added.

Pro EU-integration countries were the most critical of Britain.

"We are not on Facebook where the status is 'complicated,'" Luxembourg Prime Minister Xavier Bettel said. "Either you are married or divorced, but not something in between."

"I don't intend to accept that we are subjected to blackmail inflicted by Britain," added his Belgian counterpart Charles Michel.

But Dutch Prime Minister Mark Rutte said there was no point in giving Cameron "a bollocking" as his country had "collapsed, politically, monetarily, constitutionally, and economically" after the Brexit vote.

Since Thursday, Britain has seen the pound sink to a 30-year low against the dollar and share prices tumble, suffered credit rating downgrades and is threatened with secession by Scotland, which wants to stay in the EU.

European Central Bank (ECB) chief Mario Draghi told EU leaders that Britain's departure could shave 0.3 to 0.5 percentage points from eurozone growth over the next three years, an EU official said.

Draghi predicted "substantially lower growth in [Britain] with a possible negative spillover" to the rest of the world, but that forecast "was less negative than we expected before the Brexit," Tusk said.

Amid uncertainty over its next steps, Britain was warned that it cannot expect easy access to EU membership benefits once it walks out.

"Whoever leaves the family can't expect the same privileges as it had before without also having the obligations," Merkel said at a pre-summit parliament debate in Berlin.

EU leaders exchanged views with Cameron over dinner, after clearing other summit agenda items - including migration and foreign policy - and a meeting with NATO Secretary General Jens Stoltenberg.

The summit was due to resume Wednesday without Cameron, for the remaining 27 leaders to discuss common Brexit responses and ways to strengthen the EU. Another Britain-free summit was announced for September in Slovakia to pursue those discussions.

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