Donald Tusk advised PM Theresa May that it would be the sooner the better for Article 50 to be put into effect
European Council President Donald Tusk told Theresa May "the ball is now in your court" as the pair discussed Britain's exit from the European Union in Downing Street.
In a sign that Brussels is waiting for Mrs May to clarify when she will invoke Article 50 - the formal mechanism for exiting the bloc - Mr Tusk said it was a "crucial time" for the UK and the EU and that he wanted the process to start "as soon as possible".
The leaders of the other 27 EU nations will hold talks in Slovakia next week and Mr Tusk said they would "discuss the political consequences of Brexit" for Europe.
But he told the PM: "It doesn't mean that we are going to discuss our future relations with the UK in Bratislava, because for this - and especially for the start of the negotiations - we need the formal notification, I mean triggering Article 50. This is the position shared by all 27 member states."
"To put it simply, the ball is now in your court. I'm aware that it is not easy but I still hope you will be ready to start the process as soon as possible. I have no doubt that at the end of the day our common strategic goal is to establish the closest possible relations."
Mrs May said she wanted a "smooth" Brexit process and told Mr Tusk they had "serious issues" to discuss.
The meeting in Downing Street comes as the Prime Minister faces claims she is being vague and contradictory over the Government's position on staying in the European single market.
Mrs May has insisted she will not reveal her negotiating hand "prematurely" and won't give a "running commentary" on Brexit talks.
Earlier this week she rebuked Brexit Secretary David Davis for suggesting that continued membership of the free trade zone was "very improbable".
Meanwhile, a former top EU lawyer has told Sky News he thinks the UK will suffer economically for at least 15 years as a result of voting to leave the EU in June's referendum.