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
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"
The UK must now invoke Article 50 of the EU Lisbon Treaty, which sets out a two-year timetable for negotiations on withdrawal.
European Commission head Jean-Claude Juncker said the EU-UK split was "not an amicable divorce", but nor had they had a "deep love affair". He has also said exit negotiations should begin immediately.
"Britons decided yesterday [Thursday] that they want to leave the European Union, so it doesn't make any sense to wait until October to try to negotiate the terms of their departure," Mr Juncker said in an interview with Germany's ARD television network.
Germany wakes up to Brexit, by Jenny Hill, BBC News, Berlin
"OUTsch!"; "Well done little Britain"; "Completely detached".
Germany's press reflects the widespread shock at the British decision to leave the EU. At the highest level of government, no-one really expected this. And they are scrambling to defend German and European interests.
The German position is focused on two major, and delicate, decisions.
Firstly, whether to use Brexit as an opportunity to deepen European integration, or to view it as a "wake-up call" and create more flexibility within the union.
And, secondly, how to deal with Britain - an important trading partner - as a "third country".
Senior economists and business leaders warn against barriers to free trade.
But politicians are talking tough. Concessions, they say, might encourage other member states to leave. For this reason one senior MP told me: "There must be consequences for Britain"..
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