European Council President Donald Tusk congratulated Theresa May on her reappointment on Friday as British prime minister and urged her to start talks as soon as possible on Britain's exit from the EU to minimise disruption.

"Our shared responsibility and urgent task now is to conduct the negotiations on the UK's withdrawal from the European Union in the best possible spirit, securing the least disruptive outcome for our citizens, businesses and countries after March 2019," said Tusk, who chairs summits of European Union leaders.

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"The timeframe set by Article 50 of the Treaty leaves us with no time to lose," he added, referring to a deadline of March 2019 when Britain will no longer be a member of the bloc, whether or not a deal is reached to avoid legal uncertainties.