The leaders of Germany, France and Italy have said there can be no negotiations with Britain on its departure from the European Union until it has formally declared its intention to quit.
German Chancellor Angela Merkel said "we agree there will be no formal or informal talks" with Britain until Article 50 of the EU treaty has been invoked.
She was speaking in Berlin after meeting with French President Francois Hollande and Italian premier Matteo Renzi.
Earlier, Ms Merkel said she has a "certain amount of understanding" for the fact that Britain may need "a certain amount of time" to analyse what happens next.
But she would not comment on whether it is acceptable for Britain to wait until a new PM is in place, following David Cameron's decision to step down in the wake of the referendum result.
Mr Renzi has already warned that Brussels cannot afford to spend a "year on procedures" for Britain's exit.
Briefing his country's Senate earlier on Monday, he noted that the EU "spent a year on negotiations" aimed at satisfying Britain ahead of last week's referendum.
The EU summit this week on Britain's departure "won't be the last", Mr Renzi said, but he added that those meetings must concentrate on "the relaunching of Europe, not just procedures".
He said "pluck, lucidity and intelligence" are needed by European leaders, adding that now is not the time for improvisation.