The door is open for Britain to rejoin the EU "any time", Michel Barnier said on Wednesday - before warning the UK not to tear up Brussels regulations after Brexit.
Mr Barnier, the former Brexit negotiator, said diverging too far from EU rules now would make it more difficult for Britain to rejoin the bloc in future.
"The door to the EU side will remain open any time for you," he said. "Though everybody knows the conditions."
Those conditions would be accepting the four freedoms of the Single Market, which include freedom of movement. Joining the EU is a lengthy process that also requires candidate countries to commit to one day joining the euro.
"It will be your choice, your sovereign choice, exactly as Brexit has been the choice of the UK people, part of the UK people," he said.
Leaving the EU's single market and customs union has freed the Government to change rules inherited from the UK's EU membership, but Mr Barnier said: "My only advice is just on divergence. If there is too large divergence, it will be more difficult."
He stressed that, like Brexit, any decision to rejoin the EU would have to be a British choice alone. He said he still regretted the UK's decision to leave but added: "The EU is no longer what you left."
Mr Barnier listed changes in the EU, such as the joint borrowing of huge sums to pay for the pandemic recovery, as examples of how the bloc had integrated more closely since Brexit.
Although he admitted there was still too much bureaucracy, he claimed the EU was starting to learn the lessons of Brexit and was "less naive".
"Brexit is a failure for the EU," he said at an event at the UK In a Changing Europe think tank in London. "There is no advantage for anybody. Brexit is a lose-lose game."
The French politician, who revealed that he never though the campaign for a second Brexit referendum would be successful, said that, unlike Britain, the rest of Europe had moved on from Brexit. He added that he was surprised it was still making UK headlines.
He said he was optimistic that the UK and EU would reach a deal to cut the number of Irish Sea border checks caused by the Northern Ireland Protocol in ongoing negotiations over the treaty, which he agreed with Lord Frost.
"It seems to me that there is a common will to to progress to find operational solutions," he added.
Mr Barnier said he did not think the Brexit negotiations would have gone differently if they had been held during the war in Ukraine.
On Wednesday, Guy Verhofstadt, the European Parliament's former Brexit coordinator, said Vladimir Putin might not have invaded Ukraine had it not been for Brexit.