There can be no talks on Brexit before the UK formally begins the process to quit the EU, Germany's government says.
The UK would be given a "reasonable amount of time", but there must be no impasse, a spokesman said.
The leaders of Germany, France and Italy meet later in Berlin, with the speed of negotiations for Britain's exit high on the agenda.
UK Finance Minister George Osborne has issued a statement to try to calm markets, amid continuing volatility.
UK shares have remained uneasy in the wake of the vote, although Osborne's statement appeared to have helped lessen further big falls in stocks and the pound.
Last Thursday, Britain voted by 52% to 48% to leave the EU.
What has Germany said ahead of Monday's meetings?
Government spokesman Steffen Seibert said: "Only when Britain has made the request according to Article 50 will the European Council draw up guidelines in consensus for an exit agreement.
"One thing is clear: before Britain has sent this request there will be no informal preliminary talks about the modalities of leaving."
He added: "If the UK government needs a reasonable amount of time to do that, we respect that," but said that the uncertainty could not continue forever.
German Chancellor Angela Merkel, French President Francois Hollande and Italian PM Matteo Renzi will meet in Berlin later.
Merkel has indicated the need for calm negotiations and said she "would not fight now for a short timeframe" for Brexit.
France and Germany have insisted they are in "full agreement" on Brexit, although French Finance Minister Michel Sapin said on Monday this meant Britain should "go quickly".
Article 50 of the Treaty of Lisbon sets a two-year timetable for an exit deal. But UK PM David Cameron, who will step down by October, says he will leave the timing of its invocation to his successor. He is due to make a special address to parliament later.
What did UK chancellor George Osborne say?
After disappearing from view since the referendum vote, the chancellor, who backed Remain, tried to reassure financial markets that the UK was in a strong position to tackle the inevitable volatility.
Despite suggesting before the vote that an emergency budget would be needed, he indicated that this would not now be an immediate priority, preferring to leave any adjustments to the economy to the new PM.
He did insist he had thoroughly prepared for the contingency of Brexit, along with the Bank of England, saying: "We are equipped for whatever happens".
He also appeared to rule out resigning in the near future.
And Boris Johnson?
The leading light of the Leave campaign used an article in the Daily Telegraph to try to soothe British fears.
"EU citizens living in this country will have their rights fully protected, and the same goes for British citizens living in the EU. British people will still be able to go and work in the EU; to live; to travel; to study; to buy homes and settle down," he said.
He also suggested the UK would still have access to the EU's single market, a remark quickly challenged by the German Business Institute and Merkel ally Michael Fuchs, MP.
Fuchs said: "It will be possible, of course, but not for free - you have to see with Norway, with Switzerland, you have to pay a certain fee. And the per capita fee of Norway is exactly the same as what Britain is now paying into the EU. So there won't be any savings."
What's the latest political fall-out in the UK?
Labour faced more turmoil, with another five shadow ministers resigning on Monday, joining the 12 shadow cabinet ministers who quit the day before. Deputy head Tom Watson told Jeremy Corbyn that the party leader had "no authority" among Labour MPs.
Corbyn has announced a new team but faces a possible no-confidence vote. Nevertheless, he says he will stand in any new leadership contest.
One MP, Stephen Kinnock, accused him of leading a "half-hearted and lacklustre" referendum campaign.
Nicola Sturgeon, the first minister of Scotland, which voted 62% for Remain, told the BBC that the Scottish parliament could try to block the UK's exit from the EU.
She has also confirmed a second Scottish independence referendum was back on the table.