The European Union will not hold informal talks with the UK until it triggers Article 50 to leave, Germany, France and Italy have insisted.
German Chancellor Angela Merkel hosted talks with French President Francois Hollande and Italian Prime Minister Matteo Renzi in Berlin.
The leaders called for a "new impulse" to strengthen the EU.
Last Thursday, British citizens voted 52-48 in favour of leaving the EU in a historic referendum.
UK financial markets remain volatile in the wake of the vote, with sterling plunging to a 31-year low against the dollar, and some share trading temporarily halted.
Together with the UK, Germany, France and Italy have the largest economies in the EU.
'No further steps'
"We are in agreement that Article 50 of the European treaties is very clear - a member state that wishes to leave the European Union has to notify the European Council," Mrs Merkel told a joint news conference at the German chancellery.
"There can't be any further steps until that has happened. Only then will the European Council issue guidelines under which an exit will be negotiated.
"That means that, and we agree on this point, there will be neither informal nor formal talks on a British exit until the European Council has received the [UK's] request for an exit from the European Union."
President Hollande and Prime Minister Renzi emphasised the need to process the UK's exit as quickly as possible and focus on the challenges facing the remaining 27 states.
"Our responsibility is not to lose time in dealing with the question of the UK's exit and the new questions for the 27," Mr Hollande said. "There is nothing worse than uncertainty."