Despite the EU struggling to fight off the demands from separatist states, Mr Barnier claimed the bloc is as strong as ever when he spoke at a conference hosted by the Italian banking association.
Brussels’ chief Brexit negotiator said he has his doubts about the US commerce secretary Wilbur Ross calling on the UK to shake off the shackles of Europe so Theresa May can build better trade ties with other countries.
Mr Barnier said: “When I hear the US Trade Secretary Wilbur Ross calling in London for Brits to diverge with Europe to better converge with others — towards less environmental, sanitary, food but also probably financial, taxation and social regulation — I am left wondering.
“The UK decided to leave the EU. But will it also move off the European model? This is another question. There is behind the European regulatory framework fundamental societal choices we hold dear: the European social market economy, health protection, food safety, a fair and efficient financial regulation.”
Mr Barnier said in his speech that Brexit has united the other 27 member states saying there is a “collective awareness and a sense of responsibility in the face of the gravity of the challenges ahead and what is at stake”.
The EU chief said: “We need to continue on this path without losing sight of the essential thing: the future of the Union, which is much more important than Brexit.”
Mr Barnier said in the speech in Rome that Europe woke up “with a sense of disbelief” when the UK voted to leave the bloc.
He said: “We could hardly believe that the British people had decided, in a sovereign vote, to put an end to 44 years of common history.”
However, he said that following the referendum, Europeans expressed their desire to continue to move forward together.
He said that Europe’s response to terrorism was “proof that the things that unite us – our common values, our desire to be together and the benefits of being together – are stronger than anything that might divide us”.
Mr Barnier said the the rise of populism must not be trivialised, but said it would derail the European project. sentiment.
The EU negotiator has told the UK “time is pressing” to get a deal on a divorce bill, as Brexit talks have resumed in Brussels.
At the end of his speech, he said firmly that it is not possible to be half in and half out of the single market.
He said: “There is no reason – and I repeat, no reason – why the single market should be weakened when a member state leaves.”