'Size matters', EU warns UK as it leaves

"[…] The member states of Europe will continue to join forces and build a common future. In an age of great power competition and turbulent geopolitics, size matters," they said after returning from a retreat in France to talk about the EU's future.

"No country alone can hold back the tide of climate change, find the solutions to the digital future or have a strong voice in the ever-louder cacophony of the world. But together, the European Union can," they argued.

The three leaders argued that Brexit has strengthened EU unity.

"The last few years have brought us closer together – as nations, as institutions and as people," the presidents said three-and-a-half years after the British referendum.

"How much stronger we are when we are together," they added.

Unique Union

The EU marked the UK's departure with a sombre tone. Attention will now, however, turn to deeply-divisive issues within the EU, the negotiations over the next long-term budget, how to manage migration and how to create a more assertive EU in the world.

The talks with the UK on the future relations will also test the remaining 27 member states' unity, while a new post-Brexit power dynamic among EU countries will still have to crystallise.

At a press conference on Brexit day, Michel said that member states "will devote all our energy to a stronger and more ambitious European Union".

Von der Leyen said during the almost 50 years of UK membership, the EU "has gained political impetus, and has become a global economic powerhouse".

"Our experience has taught us that strength does not lie in splendid isolation but in our unique union," the commission chief said.

"Let there be no doubt that the challenges that EU faces and the opportunities it can grasp have not changed because of Brexit," she said.

The three institutional leaders have said they have learnt from Brexit, with Michel saying more attention needs to be paid to what citizens want and the daily added value of the EU needs to be tangible.

The three politicians spent Thursday in France at the Jean Monnet Museum to discuss the planned two-year reform exercise, the Conference on the Future of Europe.

The three institutional leaders also had a warning for the departing UK.

"We want to have the best possible relationship with the UK but it will never be as good as membership," von der Leyen said.

Michel repeated earlier warnings to London that the more the UK will diverge from EU standards the less the access to the single market.

The three leaders wrote that "without the free movement of people, there can be no free movement of capital, goods and services. Without a level playing field on environment, labour, taxation and state aid, there cannot be the highest quality access to the single market.

Talks on the future relationship are expected to start in early March with Britain, which the three have described as the EU's "natural allies".