The president of the European Council has revealed he still wants Britain to be part of the EU.Read the full story ›
Canada's free trade deal could be the last for the EU, the bloc's President Donald Tusk said.
The CETA deal with Canada is being delayed after one regional parliament in Belgium vetoed it and voiced concerns over some its terms.
Arriving for the summit, which the prime minister is also attending, Tusk said Brexit will not be discussed.
Trade will be a major issue at the two-day EU summit in Brussels, which begins today.
Tusk warned the CETA deal risked being the last such accord for the bloc unless it improves its trade policies.
There is no sense Britain is under pressure to leave the EU quickly, Theresa May's spokeswoman said after the prime minister met with Donald Tusk.
The first meeting between Mrs May and the European Council president since she became prime minister was described as "friendly", with both agreeing they should work together to ensure a "smooth" Brexit process.
Mrs May felt the EU understood her need to take time to form a negotiating stance before triggering Article 50, the formal divorce procedure, her spokeswoman said.
"The main points that the prime minister made were about working together so that there was a smooth process for the UK leaving the European Union, that is why we are taking time to prepare for the negotiations," the spokeswoman told reporters.
Mrs May also told Mr Tusk Britain would be a "strong player" while it remained in the EU and would continue to stand firm on sanctions against Russia over its action in neighbouring Ukraine.
European Council president Donald Tusk has urged Theresa May to start the Brexit process "as soon as possible", saying: "the ball is now in your court".
The leaders of the other 27 EU nations will hold talks in Bratislava next week and Mr Tusk said they would "discuss the political consequences of Brexit" for Europe.
But as they met in Downing Street, he told Mrs May: "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.
"I'm aware that it is not easy but I still hope you will be ready to start the process as soon as possible."
The prime minister said she wanted a "smooth" Brexit process and told Mr Tusk they had "serious issues" to discuss.
Poland's ruling Law and Justice party leader Jaroslaw Kaczynski has blamed European Council President Donald Tusk for Britain's vote to leave the EU, and called for him to disappear from European politics.
Mr Kaczynski told local media: "A particularly dark role was played by Donald Tusk, who conducted negotiations with the British and in fact contributed to them getting nothing.
"Hence, he is directly responsible for Brexit and should simply disappear from European politics. But this concerns the whole of European Commission in its present composition."
Donald Tusk, the president of the European Council, said Brexit was an "historic moment" but "not a moment for hysterical reactions".
He said the EU was prepared for this "dramatic scenario".
Mr Tusk added: "We are determined to keep our unity as 27."
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The incoming president of the European Council, Donald Tusk, has pledged to address the concerns of Britain in his new role.
He said the EU and he personally would take on the concerns voiced by Britain about the EU. "No reasonable person can imagine the EU without the UK," he said.
Prime Minister David Cameron has promised a referendum on continued EU membership by 2017.