Tusk says he can't pretend he is happy as formal exit begins
EU Council President Donald Tusk has said the UK is already being missed after receiving the official letter triggering Article 50, adding: "thank you and goodbye".
Mr Tusk said that the invoking of Article 50 was not a happy occasion and that the two-year negotiation ahead would be a matter of "damage control".
Speaking just minutes after Prime Minister Theresa May confirmed the delivery of the letter to MPs in the House of Commons, Mr Tusk said it was not "a happy day" for him or for the European Union.
He promised to begin arrangements for an "orderly withdrawal" for the UK, but said there was nothing for either side to gain from the two years of negotiations to come.
Mr Tusk was himself the first to announce officially that the so-called Article 50 letter had been handed to him in his Brussels office by UK permanent representative Sir Tim Barrow, revealing the news in a tweet several minutes before Mrs May's statement to the Commons began.
He added: "There is nothing to win in this process - and I am talking about both sides."
"In essence, this is about damage control."
President Tusk has convened the European Council on April 29, 2017.
In a statement the European Council said: "The European Council received a letter from the British Prime Minister, Theresa May, notifying the United Kingdom's intention to leave the European Union. This notification follows the referendum of 23 June 2016 and starts the withdrawal process under Article 50 of the Treaty. 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."
How historic day unfolded: