While prime minister Theresa May talked up Brexit opportunities in London, and there was resigned relief reigned across Europe at the start of divorce proceedings, Mr Tusk said Britain’s looming departure had made 27 member states “more determined and more united than before” – in the immediate future and beyond.

In a nod to Britain, he said: “What can I add? We already miss you.”

The European Council said in a statement it would approach the looming talks “constructively” and hoped the UK would remain as a “close partner”. German chancellor Angela Merkel echoed that sentiment, promising “fair and constructive talks” and hoped that “the British government approaches talks in this spirit, too”.

But, in a hint of tensions to come, she dismissed British hopes for negotiations on a new bilateral trade deal to run in parallel to Brexit talks. Only after the terms of separation are sealed, she said, “can we talk about our future relationship”.

The EU, led by European Commission negotiator Michel Barnier, now has to agree with London negotiators the guidelines for the looming “orderly withdrawal”.

Regardless of the Brexit talks outcome, European Parliament president Antonio Tajani remained optimistic that a “close co-operation” with Britain would be possible on “defence, police, intelligence and action against terrorism”.

“We want to support the UK as the UK has supported France and Germany,” he said, a nod to recent terror attacks. Behind the scenes, however, his officials warned there would be no trade-off of security co-operation in exchange for a trade deal.

EU founding member France conceded the move had “lifted a taboo” against what until now had been an irreversible European integration project. Days after the remaining 27 members marked the 60th anniversary of what would become the EU, French foreign minister Jean-Marc Ayrault said the start of the two-year process “has the advantage of bringing clarity”.

European security

“We don’t want them to lose their rights in the future,” he added. Yesterday’s move leaves four million people – including a million Britons – living in limbo across the continent.

After talks with commissioner Barnier on Tuesday, representatives of Britons living in Europe demanded that the first priority of Brexit talks must be an agreement to end the “suffering and uncertainty” of all EU expats.