European leaders are meeting in Berlin today to assess the repercussions of Thursday's vote
Jean-Claude Juncker, the European Commission president, says there is no reason to wait until David Cameron is replaced in October to begin negotiating Britain's exit from the European Union.
Speaking to Germany's ARD television station, Mr Juncker added that he wants to start negotiating the UK's reformed relationship with Europe "immediately".
"Britons decided (on Thursday) that they want to leave the European Union, so it doesn't make any sense to wait until October to try to negotiate the terms of their departure," he said.
"I would like to get started immediately."
It comes as the EU's six founding members - Germany, France, Luxembourg, the Netherlands, Italy and Belgium - are meeting in Berlin today to discuss Europe's future without Britain.
Mr Juncker said the separation was "not an amicable divorce", adding that "it was not exactly a tight love affair anyway".
The Prime Minister, who announced his resignation yesterday in an emotional speech at Downing Street, has said he will leave it to his successor to invoke Article 50 of the Lisbon Treaty.
The article begins the two-year process of negotiating a new trade relationship with the UK's former partners.
But Mr Juncker and other EU chiefs have urged Britain to start negotiations to quit the bloc "as soon as possible".
"Any delay would unnecessarily prolong uncertainty," Mr Juncker said in a joint statement issued with EU President Donald Tusk, EU Parliament leader Martin Schulz and Dutch premier Mark Rutte.
"We now expect the United Kingdom government to give effect to this decision of the British people as soon as possible, however painful that process may be."
The statement added: "We have rules to deal with this in an orderly way.
"We stand ready to launch negotiations swiftly with the United Kingdom regarding the terms and conditions of its withdrawal from the European Union."
It added that Britain remained bound by EU law "until it is no longer a member".
Mr Schulz also warned that the EU would not be held "hostage" while the Tory party squabbled over its next leader.
He insisted that uncertainty was "the opposite of what we need", adding that it was difficult to accept "a whole continent is taken hostage because of an internal fight in the Tory party".