The European Commission president has wished Britain well with Brexit, but warned that "strength does not lie in splendid isolation"

On the day of the UK's departure, Ursula von der Leyen and other top officials told a gathering in Brussels that they should look at the future of the European project, as the bloc plans to reinvent itself at 27 members.

“We know very well that as the sun rises tomorrow a new chapter for our union of 27 will start,” Ms von der Leyen told an audience in the Parliamentarium, a museum dedicated to the history of pan-European democracy.

“Our experience has taught us that strength does not lie in splendid isolation, but in our unique union. Nowhere else in the world can you find 27 nations of 440 million people speaking 24 different languages, relying on each other, working together, living together. "This is not by accident or by chance; this is grounded in centuries of shared history, decades of shared experience."

She added: "If you look at the enlargement process you see that the beauty of the European Union is that nobody has been forced to join the European Union. All came voluntarily."

Asked what the EU was losing with the UK's departure, she said: "We are losing a former member that was very pragmatic, very down to earth, very clear in the economic agenda. It was not always easy, but all 27 are not always easy."

Turning to trade, she reiterated the EU's long-held stance that the trade deal the UK was about to try and negotiate could not be as good as membership, adding: "Britain will become a third country, and for all third countries, the following applies: Only those who acknowledge the rules of the common market can benefit fully from the common market."

Charles Michel, the president of the European Council, told the same event: "It's never a happy moment when someone leaves but we are opening a new chapter and we will devote all our energy to building a stronger and more ambitious European Union."

He continued: "I'll say this is an exceptional, a special day for the European project. Of course, we have lukewarm feelings about what is happening, but what is the dominant feeling for me is that it's important to look to the future of the European project."

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(Photo by Glyn KIRK / AFP) (Photo by GLYN KIRK/AFP via Getty Images) GLYN KIRK AFP via Getty Brexit day: UK says goodbye to EU A man wears a pro-Brexit t-shirt Reuters Brexit day: UK says goodbye to EU Anti-Brexit demonstrators visit Europe House to give flowers to the staff on Brexit day Reuters Brexit day: UK says goodbye to EU Pro Brexit supporter wears a novelty Union Jack top hat outside the Houses of Parliament Getty Images Brexit day: UK says goodbye to EU Customers Scott Jones and Laura Jones at the Sawmill Bar in South Elmsall, Yorkshire, where a Brexit party is being held throughout the day PA Brexit day: UK says goodbye to EU A pro-Brexit demonstrator clenches his fist during a rally in London, Friday, Jan. 31, 2020. Britain officially leaves the European Union on Friday after a debilitating political period that has bitterly divided the nation since the 2016 Brexit referendum. (AP Photo/Alberto Pezzali) Alberto Pezzali AP Brexit day: UK says goodbye to EU LONDON, ENGLAND - JANUARY 31: Pro-EU activists protest at Parliament Square as people prepare for Brexit on January 31, 2020 in London, United Kingdom. At 11.00pm on Friday 31st January the UK and Northern Ireland will exit the European Union 188 weeks after the referendum on June 23rd 2016. (Photo by Jeff J Mitchell/Getty Images) Jeff J Mitchell Getty Brexit day: UK says goodbye to EU LONDON, ENGLAND - JANUARY 31: Pro Brexit supporters hold up placards at Parliament Square as people prepare for Brexit on January 31, 2020 in London, United Kingdom. At 11.00pm on Friday 31st January the UK and Northern Ireland will exit the European Union 188 weeks after the referendum on June 23rd 2016. (Photo by Jeff J Mitchell/Getty Images) Jeff J Mitchell Getty Images Brexit day: UK says goodbye to EU Pro-EU activists protest Getty Images Brexit day: UK says goodbye to EU A pro-Brexit demonstrator burns a European Union flag AP Brexit day: UK says goodbye to EU Pro Brexit supporters Getty Images Brexit day: UK says goodbye to EU Pro Brexit supporters Getty Images Brexit day: UK says goodbye to EU A Brexit supports holds a sign in Parliament Square AP Brexit day: UK says goodbye to EU A man carries an EU themed wreath Reuters Brexit day: UK says goodbye to EU Ann Widdecombe reacts with other members of the Brexit party as they leave en masse from the European Parliament PA Brexit day: UK says goodbye to EU Anti-Brexit demonstrators in Parliament Square PA Brexit day: UK says goodbye to EU Pro EU supporters let off flares from Westminster Bridge Getty Brexit day: UK says goodbye to EU British MEPs Jonathan Bullock, holding the Union Jack flag and Jake Pugh leave the European Parliament, in Brussels on the Brexit day AFP via Getty Brexit day: UK says goodbye to EU Newspapers and other souvenirs at a store, near Parliament Square Reuters Brexit day: UK says goodbye to EU Brexit supporters hold signs in Parliament Square AP Brexit day: UK says goodbye to EU Pro-EU protesters hold placards in Parliament Square AFP via Getty Brexit day: UK says goodbye to EU French newspapers PA Brexit day: UK says goodbye to EU Sinn Fein leader Mary Lou McDonald with a Border Communities Against Brexit poster before its unveiling in Carrickcarnon on the Irish border PA Brexit day: UK says goodbye to EU National growers organisation British Apples & Pears has renamed a British apple to EOS, the Greek goddess of dawn, to commemorate Brexit day AP Brexit day: UK says goodbye to EU Pro-EU protesters hold placards in Parliament Square AFP via Getty Brexit day: UK says goodbye to EU Britain's departure from the European Union was set in law on January 29, amid emotional scenes, as the bloc's parliament voted to ratify the divorce papers. 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The intervention is part of Brussels' strategy to seize the narrative by talking about the EU's future, as Britain departs. The bloc plans to hold a conference on its future later this year – with Europe Day, 9 May, a mooted date.

David Sassoli, the president of the elected European Parliament, said: "This day will go down in the history of the European Union but it will allow our institutions and our countries to be fare more united in our defence of this great vital area and the citizens of the old."

But turning to eurosceptics, he asked: "Why are these people working so hard to break up the European area? Because within our countries too, there are certain people who are trying to run with this torch. They are groups, forces, who are trying to weaken the European Union. But what's their agenda?"

Our experience has taught us that strength does not lie in splendid isolation, but in our unique union. Ursula von der Leyen, European Commission president

In a likely reference to far-right politicians like Matteo Salvini in his home state of Italy, Mr Sassoli said such people were motivated to attack the EU "because they are afraid of a rule-governed world".

Over in Dublin, Taoiseach Leo Varadkar said Ireland would take a tough stance in trade talks. Mr Varadkar is in the middle of an election campaign and facing reelection.

"Having control of their own fishing grounds is not going to be much use to them unless they can export to the EU," he told public broadcaster RTÉ.

"That is why it gets tied up with trade and we are going to take a very firm line on that to make sure that we protect our coastal communities and our fishing industry."