European Union officials have called for the UK to start the exit process as quickly as possible after Britons voted to leave the 28-member bloc, prompting the resignation of David Cameron, the prime minister, and dealing the biggest blow to European efforts at greater unity since World War II.

The outcome of Thursday's EU referendum - a 52-to-48 split in favour of Britain's exit - caused financial markets to fall sharply and brought the British pound down to a 31-year low, its biggest drop in history.

Inside Story - What's the future of the UK in the wake of Brexit?

There are now fears that the vote could set off a chain reaction of further breakaway bids by other EU members battling hostility to Brussels.

There are also worries the outcome could lead to the break-up of the UK itself after Scotland raised the prospect of another independence vote.

On Saturday, foreign ministers from the six founding members of the EU - Germany, France, Italy, the Netherlands, Belgium and Luxembourg, held a crisis meeting in Berlin to discuss the shock exit.  

"The process after Article 50 has to be initiated ... as soon as
possible to avoid a prolonged deadlock," Frank-Walter Steinmeier, the German foreign minister, said after
meeting his counterparts from France, Italy and the Benelux countries
- the founding members of the EU.

"Negotiations have to go quickly in the common interest," said Jean-Marc Ayrault, France's foreign minister, adding that the pressure would be "very strong" on Cameron at an EU summit on Tuesday to speed up the process.

Steinmeier said Britain must swiftly invoke Article 50 of the Lisbon Treaty, which guides member states wishing to leave the EU.

European leaders weigh Brexit aftermath

"The process after Article 50 has to be initiated ... as soon as possible to avoid a prolonged deadlock," he said.

His call came a day after EU chiefs also issued a joint statement saying negotiations over the so-called Brexit should begin immediately.

"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," said Donald Tusk, EU president; Jean-Claude Juncker, European Commission chief; Martin Schulz, European Parliament head; and Mark Rutte, Dutch prime minister, whose country holds the six-month EU presidency.

"Any delay would unnecessarily prolong uncertainty."

German Chancellor Angela Merkel called the result a "blow" to Europe, while French President Francois Hollande admitted it was a "grave test".

Merkel and Hollande will meet Matteo Renzi, Italy's prime minister, in Berlin on Monday to discuss future steps.

READ MORE: What will happen after Brexit?

US President Barack Obama, who publicly threw his weight behind British EU membership during a visit to London in April, insisted the "special relationship" between the two countries was "enduring".

Following the UK's vote in favour of exiting the bloc, Dutch far-right MP Geert Wilders and French National Front leader Marine Le Pen called for referendums on EU membership in their own countries immediately.

The British vote will lead to at least two years of divorce proceedings with the EU, the first exit by any member state.

Cameron, who led the campaign to remain in Europe to defeat, after promising the referendum in 2013, said he would resign by October and it would be up to his successor to formally start the exit process.

Cameron says it is up to his successor formally to start the exit process [Stefan Wermuth/Reuters]

"The British people have made the very clear decision to take a different path and as such I think the country requires fresh leadership to take it in this direction," Cameron, in office since 2010, said in an emotional televised address outside his residence on Friday.

"I do not think it would be right for me to be the captain that steers our country to its next destination."

Yet, Juncker said he did not understand why the British government needed "until October to decide whether to send the divorce letter to Brussels.

"I'd like it immediately," the European Commission chief told the German broadcaster ARD on Friday evening.

"It is not an amicable divorce but it was also not an intimate love affair," he added.

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Cameron's Conservative Party rival Boris Johnson, the former London mayor who became the most recognisable face of the Leave camp, is now widely tipped to seek his job.

"We can find our voice in the world again, a voice that is commensurate with the fifth-biggest economy on earth," Johnson said.

The "Leave" victory also threatens to shatter the unity of the UK, with Scotland and Northern Ireland voting to remain in while England - barring big cities like London - and Wales supported out.

Nicola Sturgeon, Scottish first minister, said a second independence vote was now "highly likely" after a 2014 referendum backed staying in the UK.

"The option of a second referendum must be on the table and it is on the table," she said, declaring it was "democratically unacceptable" for Scotland to be dragged out of the EU against its will.

The Scottish parliament is due to meet for an emergency session early on Saturday.

Sturgeon said a second Scotland independence vote was now 'highly likely' [Reuters]

In Northern Ireland, the nationalist Sinn Fein party seized on the result to call for a vote on reunification with the Irish Republic.

The possibility of such a vote is included in the 1998 peace accord that largely ended three decades of violence in the province, but leaders north and south of the border were quick to dismiss the idea.

Highlighting the discord, a petition demanding a second EU referendum had gathered more than one million signatures on Saturday morning.

Risks ahead

The result caused the pound to fall to a 31-year low of $1.3229 at one point but it recovered some ground after the Bank of England said it stood ready to pump £250bn ($370bn) into the financial system to avert a crisis.

European stock markets dropped around 8 percent at opening before recovering later, while British bank shares lost a quarter of their value in morning trade.

READ MORE: After Brexit, could there be Grexit?

Credit rating agency Moody's downgraded the UK's economic outlook from stable to negative, saying Britain faces "a prolonged period of uncertainty ... with negative implications for the country's medium-term growth outlook".

Huge questions also face the large numbers of British expatriates who live and work freely elsewhere in the EU, as well the fate of EU citizens who live and work in Britain.

 

Source: Al Jazeera and agencies