A statement released by the European Union said: “We regret that the United Kingdom will leave the European Union, but we are ready for the process that we now will have to follow.

“For the European Union, the first step will now be the adoption of guidelines for the negotiations by the European Council. These guidelines will set out the overall positions and principles in light of which the Union, represented by the European Commission, will negotiate with the United Kingdom.

“In these negotiations the Union will act as one and preserve its interests. Our first priority will be to minimise the uncertainty caused by the decision of the United Kingdom for our citizens, businesses and Member States. Therefore, we will start by focusing on all key arrangements for an orderly withdrawal.

“We will approach these talks constructively and strive to find an agreement. In the future, we hope to have the United Kingdom as a close partner.”

The EU statement followed its receipt of a letter from UK Prime Minister Theresa May.

President Tusk has convened the European Council on 29 April 2017. Britain will not be permitted to participate in the meeting.