Donald Tusk told the UK that the rest of the EU already misses the country just minutes after Theresa May invoked Article 50 of the Lisbon Treaty, and triggered two-year-long Brexit talks on Wednesday 29 March.
"There is nothing to win in this process and I am talking about both sides. In essence, this is about damage control," the EU Council President said at a press conference in Brussels. "We already miss you. Thank you and goodbye."
The Polish politician issued the statement after Sir Tim Barrow, the UK's chief representative to the EU, hand-delivered the Article 50 letter from May.
The symbolic document was drawn up around nine months after the UK voted to split from the economic and political bloc on 23 June 2016.
"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," Tusk added.
"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 other 27 EU leaders will meet on the 29 April to thrash out Brexit negotiation guidelines for the EU Commission, the executive arm of the bloc. Jean Claude Juncker, the EU Commission chief, will oversee the talks, but Michel Barnier will serve as the institution's chief negotiator.
The French politician has said that he expects a deal between the EU and UK to be done by October 2018, with ratification in 2019.
May's call for unity
May, meanwhile, called for UK-wide unity as she addressed the House of Commons on Wednesday.
"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 tell the House of Commons," she told MPs.
"And yes, those EU nationals who have made this country their home. It is my fierce determination to get the right deal for every single person in this country."
Labour leader Jeremy Corbyn responded by urging the Conservative premier to deliver for the "whole country, not just the few".
"We will use all means possible to make sure we hold the government to their word on full access to the single market, on protecting Britain from being dragged into a race to the bottom, and ensuring our future relationship with the EU is strong and co-operative," the left-winger said.
"A relationship where we can work together to bring prosperity and peace to our continent. If the Prime Minister can deliver a deal that meets our tests, we will back her.
"More than ever, Britain needs a government that will deliver for the whole country, not just the few. And that is the ultimate test of the Brexit deal that the Prime Minister must now secure."