Brexit: European leaders tighten screws on Britain to quickly invoke Article 50 to leave EU

Warning that a prolonged process in leaving the EU would endanger the economy, the trio – leaders of Europe's three most populous nations – insisted that Britain take the plunge immediately.

"There's nothing worse than uncertainty that generates political and financial behaviour that can be irrational," said the French President Francois Hollande following the trilateral talks in Berlin with German Chancellor Angela Merkel and Italian Prime Minister Matteo Renzi. Hollande urged the UK not to "waste time" in beginning the exit process adding that "Being responsible means not wasting time in engaging with the question of Britain's departure and setting this new impulse we want to lend the new European Union."

The three leaders held crunch talks on Monday, 27 June after the UK voted to leave the 28-nation bloc.

"The UK must invoke Article 50. Beforehand, no negotiations are possible. There will be no informal or formal discussions before Article 50 is invoked," said the German chancellor referring to the EU's Lisbon Treaty, which deals with the procedures of leaving the union. The three leaders, in a joint statement, said Europe "is strong enough to find the right answers" in the wake of the UK's decision to depart from the EU.

The EU is technically in uncharted waters as Article 50 has never been triggered before. However, the UK is still reluctant to immediately kick-start the painful process as it is still untested and no member state has left the bloc in its 60-year history.

Cameron, who earlier said a new leader would take over in October, has made it clear the onus is on the incoming leader to trigger Article 50. Chancellor George Osborne also echoed this view by saying Britain should activate the process only when it has a "clear view" of how it would set up its relations with the European Union. Cameron is also set to attend a Brussels summit, the first talks after the referendum, with European leaders later in the day where he will be put under pressure to explain Britain's situation.