A sombre-faced European Council president Donald Tusk held up the letter announcing Britain's intention to leave the EU as he delivered the sad message: "We already miss you. Thank you and goodbye."

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, which would be no more than an effort at "damage control".

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.

Mr Tusk said that the remaining 27 EU states were "more determined and more united than before", following their agreement on plans for the future at the Union's 60th anniversary commemorations in Rome last weekend.

The unity of the 27 gave him and the European Commission a "strong mandate" to protect the interests of the 27 in the "difficult" Brexit negotiations, he said.

Holding up the Article 50 letter at a Brussels press conference, Mr Tusk said: "So here it is. Six pages. The notification from Prime Minister Theresa May triggering Article 50 and formally starting the negotiations over the UK's withdrawal from the European Union.

"There is no reason to pretend that this is a happy day, neither in Brussels nor in London.

"After all, most Europeans - including almost half the British voters - wish that we would stay together, not drift apart.

"For me, I will not pretend that I am happy today."

"But, paradoxically, there is also something positive in Brexit. Brexit has made us, the community of 27, more determined and more united than before.

"I am fully confident of this, especially after the Rome declaration. Today, I can say that we will remain determined and united also in the future during the difficult negotiations ahead.

"This means that both I and the Commission have a strong mandate to protect the interests of the 27."

Mr Tusk said that the EU's clear goal in negotiations was "to minimise the cost for the EU's citizens, businesses and member states" resulting from Brexit.

"We will do everything in our power and we have all the tools to achieve this goal," he said.

The European Council president added: "As for now nothing has changed. Until the United Kingdom leaves the European Union, EU law will continue to apply to and within the UK."

An official statement released by the European Council on behalf of leaders of the 27 remaining member states stressed that "we will act as one and start negotiation by focusing on key arrangements for an orderly withdrawal", he said.

Mr Tusk said he would unveil his draft negotiating guidelines in Malta on Friday, ahead of an extraordinary summit on April 29 when they are expected to be adopted by the EU27.

Concluding his comments, he sent a personal message to Britain: "What can I add to this? We already miss you. Thank you and goodbye."