The six founding members of the European Union sent a clear message to Britain today to leave the bloc as soon as possible after Britons voted to quit in the biggest blow to the project since World War Two.

Eager to shore up the EU for its other 27 members, foreign ministers from the six founding countries pressed Britain to trigger the process for exiting the bloc so that they are not left in limbo and can concentrate shaping the future of Europe.

"We now expect the UK government to provide clarity and give effect to this decision as soon as possible," the ministers from Germany, France, Italy, the Netherlands, Belgium and Luxembourg said in a joint statement.

Separately, German Chancellor Angela Merkel said the negotiations with Britain should not be conducted in such a way as to be seen as a deterrent to other countries, and that there was no hurry for London to trigger the process for leaving.

"Quite honestly, it should not take ages, that is true, but I would not fight now for a short time frame," Merkel told a news conference at a meeting of her party outside Berlin.

There's no reason to play a cat and mouse game. That would not be respectful after deciding to organise this referendum

Yesterday, Former London mayor Boris Johnson, a leading campaigner for Britain to leave the EU and the bookmakers' favourite to replace David Cameron as prime minister, said nothing would change over the short term following the Brexit vote.

Only Britain can invoke Article 50 of the EU treaty required to set in motion the process to exit the bloc.

The foreign ministers of both France and Luxembourg warned Britain not to play games by drawing out the process.

"There's no reason to play a cat and mouse game. That would not be respectful after deciding to organise this referendum," French Foreign Minister Jean-Marc Ayrault told a joint news conference after the meeting of the six in Berlin.

"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 said.

Global stock markets plunged yesterday, and sterling saw its biggest one day drop in more than 30 years after Britons voted by 52-48 percent to exit the EU, which it joined more than 40 years ago.

Ratings agency Moody's downgraded its outlook for Britain, saying its creditworthiness was now at greater risk as the country would face substantial challenges to successfully negotiating its exit from the bloc.

Italian Foreign Minister Paolo Gentiloni and German Foreign Minister Frank-Walter Steinmeier (L-R) attend a press conference after a foreign minister meeting of the EU founding members in Berlin.

In their statement, the six foreign ministers lamented the watershed brought by the 'Brexit' vote. They said the EU was losing "not just a member state but history, tradition and experience."

The ministers also said they would have to deal with varying appetites for European integration among member states in order to meet voters' expectations.

In Colmar in eastern France, French President Francois Hollande echoed their sentiment, saying: "It will be painful for Britain but ... like in all divorces, it will be painful for those who stay behind too."

Merkel also said Britain must say what kind of relationship it wants with the EU before the bloc examines how to respond.

Ayrault, the French foreign minister, said earlier other EU leaders would press Cameron at a summit meeting next week to act quickly: "There will be a lot of pressure on Cameron on Tuesday to move ahead," he said.