There is no strength in 'splendid isolation': European Union to UK
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'Our union has gained political impetus and has become a global economic powerhouse,' she said, appearing with EU Council President Charles Michel and European Parliament speaker David Sassoli.
As Britain is set to bid farewell to the European Union, the bloc warned that the UK would lose the benefits of close ties after Brexit.
European Commission president Ursula von der Leyen said, "We want to have the best possible relationship with the United Kingdom, but it will never be as good as membership".
"Our union has gained political impetus and has become a global economic powerhouse," she said, appearing with EU Council President Charles Michel and European Parliament speaker David Sassoli.
She also said that "Our experience has taught us that strength does not lie in splendid isolation, but in our unique union."
After years of political deadlock, Britain is due to leave the European Union at 2300 GMT. However, under the 11-month of the transition period, the country will remain under EU rules for trade, travel and business. But, legally Britain is out of the bloc and will go it alone for the first time since 1973.
The phrase "splendid isolation" is associated with Britain's 19th-century foreign policy of standing aloof from the politics of the European mainland, while tending to a global trading empire.
Earlier in the day, Prime Minister Boris Johnson said, "Our job as the government, my job, is to bring this country together and take us forward."
"This is not an end but a beginning. This is the moment when the dawn breaks and the curtain goes up on a new act," Johnson said in a statement to mark the occasion.
(With inputs from agencies)