BERLIN (REUTERS) - German Chancellor Angela Merkel called on Saturday for clear-headed negotiations with "close partner" Britain over its departure from the European Union.

Foreign ministers of the EU's six founding members had earlier said Britain should leave the bloc as soon as possible after Britons voted on Thursday to quit the 28-member bloc.

However, Merkel struck a more conciliatory tone.

"The negotiations must take place in a businesslike, good climate," Merkel said after a meeting of her conservative party in Hermannswerder, outside Potsdam, to the west of Berlin.

"Britain will remain a close partner, with which we are linked economically," she said, adding that there was no hurry for Britain to invoke Article 50 of the EU treaty - the first step it must take to set in motion the exit process.

"It should not take ages, that is true, but I would not fight now for a short time frame," Merkel said, in contrast with the more urgent call by the foreign ministers of Germany, France, Italy, the Netherlands, Belgium and Luxembourg, who were meeting to the north of the German capital.

They pressed for Britain to trigger the process for exiting the bloc after Britons voted by 52-48 per cent to exit the EU, which it joined more than 40 years ago.

"We now expect the UK government to provide clarity and give effect to this decision as soon as possible," they said in a joint statement.

The president of the European Parliament, Martin Schulz, also piled pressure on Britain, calling for it to trigger the Leave process at a summit of EU leaders next Tuesday.

"We expect the British government to deliver now," Schulz told Germany's Bild am Sonntag newspaper.

"The summit on Tuesday is the appropriate moment to do so."

EU officials said there was no real problem if it took a few months to begin the process for Britain to leave, though waiting until the end of the year could get in the way of the next round of EU budget talks and European election campaigning.

The officials said they were worried that British Prime Minister David Cameron, who resigned on Friday, could hand over to someone with a strategy to drag out the country's departure.

Matthew Elliott, chief executive of the Vote Leave campaign, said Britain should begin informal negotiations on a full settlement governing its post-Brexit relationship with the EU before invoking Article 50 of the Lisbon Treaty.

FRENCH PRESSURE

France also pressed for a swift start to the exit process, with Foreign Minister Jean-Marc Ayrault saying negotiations must move quickly and that the remaining EU member states also needed to give fresh impetus to the European project.

"We have to give a new sense to Europe, otherwise populism will fill the gap," he said, adding that the EU could not wait for Cameron to depart in October before the exit process begins.

Ayrault said other EU leaders would put "a lot of pressure"on Cameron at Tuesday's summit to act quickly.

French President Francois Hollande said the separation "will be painful for Britain but ... as in all divorces, it will be painful for those who stay behind, too".

France and Germany have drafted a 10-page paper mapping out three areas of immediate concern for the remaining EU members: security, migration and refugees, and jobs and growth which diplomatic sources say they want to use as a basis to shore up the EU, while building a more flexible union.

Global stock markets plunged on Friday, and sterling saw its biggest one-day drop in more than 30 years, while ratings agency Moody's downgraded Britain's credit outlook.

The six foreign ministers said the EU was losing "not just a member state but history, tradition and experience".

Luxembourg Foreign Minister Jean Asselborn told Reuters: "I believe you can destroy the European Union with referenda. We have to communicate better what the EU has done, and we have to work harder on issues such as migration where we have failed."

Both Ayrault and Asselborn warned Britain not to play games by drawing out the exit process.

"It's in Britain's interest and in the interest of Europeans not to have a period of uncertainty that would have financial consequences, and that could have economic and political consequences," he told a news conference after the meeting.