The European Union has clarified the way the UK can kickstart formal negotiations to exit the bloc following Thursday's referendum.
It says Britain can trigger Article 50, which sets a two-year deadline for a deal, by making a formal declaration either in a letter or a speech.
UK PM David Cameron has said he will step down by October to allow his successor to conduct the talks.
But EU foreign ministers have urged Britain to start the process soon.
Since Thursday's vote there has been intense speculation about when, and how, the UK might begin formal negotiations.
A spokesman for the European Council, which defines the EU's political direction and priorities, reiterated on Saturday that triggering Article 50 was a formal act which must be "done by the British government to the European Council".
"It has to be done in an unequivocal manner with the explicit intent to trigger Article 50," the spokesman said.
"It could either be a letter to the president of the European Council or an official statement at a meeting of the European Council duly noted in the official records of the meeting."
On Saturday, German Chancellor Angela Merkel said the EU had "no need to be particularly nasty in any way" in the negotiations with Britain.
She said that deterring other countries from leaving the EU should not be a priority in the talks.
Mrs Merkel added that she was not in favour of pushing for a speedy withdrawal.
"It shouldn't take forever, that's right, but I would not fight for a short timeframe," she said.
In other developments:
- UK Labour leader Jeremy Corbyn, facing a no-confidence vote following the referendum, has sacked shadow foreign secretary Hilary Benn
- Scotland's First Minister Nicola Sturgeon says she will seek "immediate discussions" with Brussels to "protect Scotland's place in the EU"
- A petition calling for a second referendum on UK's membership of the EU has gained more than two million signatures
- There are warnings British financial institutions could lose their prized access to the EU if the UK leaves the single market
- Britain's European Commissioner, Lord Hill, who oversees financial services, is to resign
Mrs Merkel was speaking after several EU foreign ministers, including Germany's, had urged Britain to quickly implement its exit.
"This process should get under way as soon as possible so that we are not left in limbo but rather can concentrate on the future of Europe," German Foreign Minister Frank-Walter Steinmeier said.
His Dutch counterpart Bert Koenders said the continent could not accept a political vacuum, saying "this will not be business as usual".
Speaking later to the BBC, Estonian President Toomas Hendrik Ilves said he didn't think it was "even legally possible" to force the UK to speed up the exit process.
"I understand it is very difficult for Prime Minister Cameron, who was against leaving the European Union, to now go ahead and do this," he told the BBC World Service's Newshour programme.
"I think we should give them time; let them decide how quickly they want to do it."
He described Britain's exit from the bloc as "a disaster" saying Estonia had often aligned itself with the UK and had counted on Britain to present their shared views.
The UK's decision to leave the EU has sent shockwaves across the continent with leaders of Eurosceptic parties in France, the Netherlands and Italy demanding referendums in their own countries.
New road map
In response, some EU politicians have called for speedy reforms to quell further unrest.
French economy minister Emmanuel Macron suggested a new mission statement should be drafted and put to a referendum of all EU citizens.
"We've never had the courage to organise a true European referendum in its real sense," he told a conference.
"We would first build this new project with European peoples and then submit this new road map, this new project, to a referendum [across the bloc]."
The first summit of EU leaders with no British representation will be held on Wednesday, a day after Mr Cameron holds talks with members.
Global stock markets and the pound fell heavily on the news of the so-called "Brexit", while credit rating agency Moody's cut the UK's outlook to "negative".
What comes next?
What is Article 50 of the EU Treaty?
- In force since 2009 but never tested
- Allows governments to notify intent to leave. Talks then begin on a range of issues between the leaving nation and other EU members
- If no deal is reached, membership will automatically cease two years after notification
- The article is only a basic template for leaving, settling the date and some other matters. It does not automatically include issues such as movement of people or trade. The latter could take years to conclude