Foreign ministers of EU's founding member states, gathering in Berlin for crisis talks after Britain's shock referendum outcome, said London must begin the process of leaving "as soon as possible".
France's Jean-Marc Ayrault went as far as to call for David Cameron, who has said he would resign by October, to make way fast for a new British prime minister to manage the transition out of the union.
As the EU grappled with the first defection in its six-decade history, European Commission chief Jean-Claude Juncker warned London against foot-dragging now that it had made its fateful choice.
"It is not an amicable divorce but it was also not an intimate love affair," he said.
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European Parliament President Martin Schulz called Cameron's decision to possibly wait until October to leave "scandalous", saying that he was "taking the whole (European) continent hostage".
It will fall to Cameron's successor to lead the complex negotiations under Article 50 of the EU's Lisbon Treaty which sets out a two-year timeframe to leave.
German Foreign Minister Frank-Walter Steinmeier, hosting his counterparts from the five other original EU states, said they agreed that London must begin the exit process immediately.
"We understand and respect the result (of the referendum) and understand that Britain is now concentrating on Britain," Steinmeier said, flanked by Ayrault, the Netherlands' Bert Koenders, Italy's Paolo Gentiloni, Belgium's Didier Reynders and Luxemburg's Jean Asselborn.
He insisted London still had a "responsibility" toward the EU.
"We must now be allowed to focus on the future of Europe and that means that after the decision taken in Britain, the process by which we negotiate Britain's exit must begin."