EU leaders issued a stark warning to Boris Johnson that shifting away from Brussels' rules will leave the UK with weakened access to the single market.
On the day the UK leaves the bloc, they made a joint statement lamenting Brexit but vowing to build a "stronger" Europe.
A spokesman for Boris Johnson, who is preparing to back a 'Canada-style' trade deal in a speech on Monday, later admitted the deal will not be "frictionless".
European Council President Charles Michel said: "The more the UK will diverge from the EU standards, the less the access to the single market they will have."
European Commission president Ursula von der Leyen agreed: "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."
She explained: "We want to have the best possible relationship with the United Kingdom but it will never be as good as membership," she said.
She quoted Jean Monnet, considered to be one of the founding fathers of the EU, saying: "We are not optimistic, we are not pessimistic, we are determined."
Speaking moments after the press conference, Boris Johnson's spokesman admitted trade with the EU would not be“frictionless” after Brexit.
He said: “You can only have fully frictionless of you accept all their rules.
“We have been clear that we will not be doing so.”
Michel, European Parliament President David Sassoli and the head of the bloc's executive arm, Ursula von der Leyen of the European Commission, delivered a statement on the future of the bloc in Brussels.
"We want to have the best possible relationship with the United Kingdom but it will never replicate the benefits of membership," von der Leyen told reporters in a joint statement.
The three leaders said the EU would now focus on fighting climate change and advancing digitalisation.
Mr Michel spoke of an "exceptional day" as the UK entered its final hours in the bloc.
"Today is an exceptional day for the European Union and today probably we have mixed feelings," Mr Michel said.
"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."
Reflecting on the loss of the UK, von der Leyen said the EU was losing a 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."
Asked about what the EU had learnt from Brexit Mrs von der Leyen said: "It is the British people who decided, so this has to be respected, but within these three-and-a-half years of Brexit there was a very precious experience made by the 27 of the European Union.
"This is the experience of how much unity counts, how strong we are in unity, way more than by 27, it's way more than by each single country will ever has on its own.
"We made the experience how strong we are in defending our values, our interests, but also protecting our member state Ireland.
"This experience of being together and solving problems together was a valuable one even if, as I say, we regret the decision of the British people but we respect it, and now we open another chapter it's a story of old friends and new beginnings."
Mr Michel discussed how the bloc will work to make sure that Europe is "better loved".
"We want them to be keener about the European project and so our concern is to stop using jargon and to cut down the red tape," he said.
It came as The Times reported Boris Johnson will back an "off-the-shelf" Canada-style trade deal mooted by EU chief negotiator Michel Barnier.
The Prime Minister is expected to give a speech on Monday saying what he would like from future trade.
Mr Johnson's spokesman said the government has been clear it is seeking a Canada-style free trade agreement with the EU.
