European Council President Donald Tusk holds the formal notice launching the Brexit as he arrives to give a press conference, in Brussels on March 29, 2017 | Aurore Belota/AFP via Getty Images Accepting Brexit notice, Donald Tusk warns EU law still applies to UK ‘There is nothing to win in this process,’ says grim-faced European Council president.

Grim-faced and making no attempt to hide it, European Council President Donald Tusk accepted the U.K.'s formal notification Wednesday and quickly warned Britain that it must obey EU law until the departure terms are settled.

"So, here it is, six pages," Tusk declared, holding the document aloft as if it were a soiled diaper, moments after accepting the letter from U.K. ambassador Tim Barrow. "The notification from Prime Minister Theresa May, triggering Article 50 and formally starting the negotiations of the United Kingdom's withdrawal from the European Union."

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

While his tone was somber and regretful, Tusk was also pointed in getting down to business, saying that his responsibility was to safeguard the interests of the EU's remaining 27 members, and his goal was to limit costs, including for citizens and businesses.

And in warning that EU law still applied to Britain, he seemed intent on cutting off any speculation that May or her government might attempt to unilaterally change immigration policy or any other rules prior to the U.K.'s official departure. The European Parliament has issued a similar warning.

"What we should stress today is that, as for now, nothing has changed," he said. "Until the United Kingdom leaves the European Union, EU law will continue to apply to — and within — the U.K."

That departure will occur on a mutually agreed-upon date or, if talks fail, on March 29, 2019, the official deadline under the EU treaties for a negotiated withdrawal. An extension is possible but only with the unanimous consent of all EU countries, including the U.K.

Tusk, in his brief, terse remarks, said that Brexit had brought the remaining EU countries closer together.

"Paradoxically there is also something positive in Brexit," he said. "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."

Still, he said the overall situation was bad. "There is nothing to win in this process," he said. "And I am talking about both sides. In essence, this is about damage control."

Tusk said that the European Council was in the process of developing negotiating guidelines to be approved at a summit of EU leaders on April 29, and that he would describe the plans in further detail at a news conference Friday with Maltese Prime Minister Joseph Muscat, whose country currently holds the EU's rotating presidency.

"We will act as one," he said, before closing on a poignant note.

"What can I add to this? We already miss you," he said. "Thank you and goodbye."