'Unhappy' day as UK delivers Brexit letter

"I am writing to give effect to the democratic decision of the people of the United Kingdom," May wrote in the letter.

The message officially launches the two-year exit procedure from the EU - based on Article 50 of the Lisbon Treaty.

May wrote that the UK wanted "a deep and special partnership that takes both economic and security cooperation".

"The task before us is momentous but it should not be beyond us," she added.

Reading a statement to journalists, Tusk said that "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 of [the] British voters wish that we would stay together, not drift apart," he insisted.

Positive paradox

"We already miss you," he told Britons, adding that "paradoxically there is something positive in Brexit".

"Brexit has made us, the community of 27, more determined and more united than before," he said, assuring that the 27 will stay "united and determined" during the two-year divorce talks.

The European Council chief warned that no one would emerge as a winner during the divorce process, which will essentially be about "damage control".

"Our goal is to minimise the cost for EU citizens, businesses and member states," Tusk said, while reiterating that both he and the European Commission have a strong mandate to protect the EU's interests.

He highlighted that even though the exit procedure can now officially start, the UK remains a member of the union until the two-year deadline is over.

"As for now nothing has changed, until the UK leaves, EU law will apply to and within the UK," he said.

EU-27 guidelines

The next step for Tusk is to draw up the negotiating guidelines for the EU, which the 27 leaders will adopt at a summit on 29 April.

Tusk said he will share his proposal for guidelines with member states on Friday.

The EU-27 also put out a statement reacting to the official launch of the UK's exit procedure.

They pledged to act as "one and preserve its interests". Their statement reiterated that EU countries will start by focusing on all key arrangements for "an orderly withdrawal".

The EU-27 said they will approach the talks "constructively and strive to find an agreement". However, contrary to an earlier draft, the statement does not touch upon the scenario where no deal is reached at the end of the two-year negotiating period.

Nevertheless, the EU-27 hope to have the UK "as a close partner" in the future.