Donald Tusk this afternoon gave an emotional farewell to Britain after Theresa May formally triggered Article 50, telling the UK: "We already miss you. Thank you and goodbye."
At a highly charged press conference in Brussels the EU Council chief looked devastated as he said Britain's formal notification of its withdrawal from the EU was a "sad day".
And he said Brexit was a "lose-lose situation" for both Britain and the EU, stressing that the bloc's principle aim in the negotiations would be "damage control".
Britain's ambassador to the EU, Sir Tim Barrow, formally handed in the Government's intention to begin divorce proceedings in an historic meeting with the EU Council president this morning
Mr Tusk tweeted a photogaph of the history defining moment the missive, hand signed by Theresa May, was served saying Britain had finally "delivered" on Brexit.
His statement was incredibly conciliatoty, and notably a phrase mentioning the prospect of the negotiations failing was cut after appearing in the initial draft.
Addressing reporters in Brussels this afternoon he said: "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."
The EU Council chief, who spoke quietly with his head bowed, said that he and Commisson chief Jean-Claude Juncker were now determined to protect the interests of the other 27 member states.
"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 and today I can say that we will remain determined and united also in the future also during the difficult negotiations ahead.
"This means that both I and the Commission have a strong mandate to protect the interests of the 27. There is nothing to win in this process and I’m talking about both sides."
"In essence this is about damage control. Our goal is clear - to minimise the cost for the EU citizens businesses and member states.
"We will do everything in our power and we have all the tools to achieve this goal and what we should stress today is that as for now nothing has changed."
Mr Tusk also insisted that Britain would remain a member of the club until its departure in Spring 2019, during which time it would remain subject to EU law.
He added: "Until the UK leaves the EU EU law will continue to apply to and within the UK.
"We will act as one and start negotiations by focussing on all key arrangements for an orderly withdrawal. On Friday I will share a proposal of the negotiating guidelines with the member states to be adopted by the European Council on the 29th of April."
And in apparently unscripted remarks, the visibly emotional eurocrat then signed off with a message for Britain, telling reporters: "What can I add to this? We already miss you. Thank you and goodbye."
Earlier, Prime Minister Theresa May will file formal Brexit divorce papers on Wednesday, pitching the United Kingdom into the unknown and triggering years of uncertain negotiations that will test the endurance of the European Union.
Nine months after Britons voted to leave, May will notify EU Council President Donald Tusk in a letter that the UK really is quitting the bloc it joined in 1973.
The prime minister, an initial opponent of Brexit who won the top job in the political turmoil that followed the referendum vote, will then have two years to settle the terms of the divorce before it comes into effect in late March 2019.
"Now that the decision has been made to leave the EU, it is time to come together," May will tell lawmakers, according to comments supplied by her office.
"When I sit around the negotiating table in the months ahead, I will represent every person in the whole United Kingdom – young and old, rich and poor, city, town, country and all the villages and hamlets in between," May will say.
On the eve of Brexit, May, 60, has one of the toughest jobs of any recent British prime minister: holding Britain together in the face of renewed Scottish independence demands, while conducting arduous talks with 27 other EU states on finance, trade, security and a host of other complex issues.
Read more: Queen signs Brexit bill into law to allow Theresa May to withdraw Britain from EU
The outcome of the negotiations will shape the future of Britain's $2.6 trillion economy, the world's fifth biggest, and determine whether London can keep its place as one of the top two global financial centers.
For the EU, already reeling from successive crises over debt and refugees, the loss of Britain is the biggest blow yet to 60 years of efforts to forge European unity in the wake of two devastating world wars.
Its leaders say they do not want to punish Britain. But with nationalist, anti-EU parties on the rise across the bloc, they cannot afford to give London generous terms that might encourage other member states to follow its example and break away.
BREXIT DEAL
May's notice of the UK's intention to leave the bloc under Article 50 of the EU's Lisbon Treaty is due to be hand-delivered to Tusk in Brussels by Tim Barrow, Britain's permanent representative to the EU.
May, who on Tuesday signed the Brexit letter and spoke to German Chancellor Angela Merkel about the future talks, will update the British parliament on Wednesday while Tusk is due to give a briefing to reporters.
EU officials expect May's notice of intention to leave the bloc under Article 50 of the EU's Lisbon Treaty to be hand-delivered by British diplomats on Wednesday, when May will
The Brexit letter will seek to set a positive tone for the talks and recap 12 key points which May set out as her goals in a speech on Jan. 17.
Within 48 hours of reading the letter, Tusk will send the 27 other states draft negotiating guidelines. He will outline his views in Malta, where from Wednesday he will be attending a congress of center-right leaders. Ambassadors of the 27 will then meet in Brussels to discuss Tusk's draft.
The course of the Brexit talks is uncertain.
May has promised to seek the greatest possible access to European markets but said Britain will aim to establish its own free trade deals with countries beyond Europe, and impose limits on immigration from the continent.
Also read: British Parliament passes Brexit bill
She has acknowledged that those measures would require withdrawing from the EU 'single market' of 500 million people, founded on the principles of free movement of goods, services, capital and people.
Her priorities also include leaving the jurisdiction of the European Court of Justice and securing "frictionless" trade with the bloc while ending full membership of the customs union that sets external tariffs for goods imported into the bloc.
She wants to negotiate Britain's divorce and the future trading relationship with the EU within the two-year period, though EU officials say that will be hard.
"It was you, the British, who decided to leave, not us who wanted you to go," said one senior EU diplomat. "The trading relationship is going to be the most difficult bit to solve - I don't see how that will be done in that time frame."
A huge number of questions remain, including whether exporters will keep tariff-free access to the single market and whether British-based banks will still be able to serve continental clients, not to mention immigration and the future rights of EU citizens in the UK and Britons living in Europe.
Global banks such as Goldman Sachs are considering moving staff out of Britain due to Brexit, and some major companies and banks could use the Article 50 trigger date to update investors on their plans.
THE UNITED KINGDOM
At home, May's United Kingdom - a nuclear power with a permanent seat on the United Nations Security Council - is divided and faces strains that could lead to its break-up.
The results of the Brexit referendum called the country's future into question because England and Wales voted to leave the EU but Scotland and Northern Ireland voted to stay.
Scottish nationalists have demanded an independence referendum that May has refused, saying the time is not right. In Northern Ireland, rival parties have been unable to end a major political crisis for over two months and Sinn Fein nationalists are demanding a vote on leaving the UK and uniting with the Republic of Ireland.
Read more: British MPs approve bill to start Brexit
"May's job is just so difficult - keeping the UK together while Brexiting - that I am not sure anyone would want it," said a senior non-EU diplomat who spoke on condition of anonymity.
"After Brexit, the future of almost everything is completely unclear and that is extremely worrying for the UK, the EU and the West as a whole."
