The former Brexit negotiator said the UK knew "exactly" what the conditions for rejoining the bloc were and that “the door is open”.
But speaking to ITV's Peston programme Mr Barnier blasted Boris Johnson's handling of talks with Brussels – accusing him of disrespecting his own signature on the deal he signed.
"The point is not to sign a treaty, the point is to respect the treaty and to respect its own signature," Mr Barnier said of the ex-PM's approach.
"I never understood, I do not still understand, how the prime minister of such a country, a great country like the UK, can be able not to respect his own signature.”
Brussels was enraged by Mr Johnson's breaking of commitments he signed up to in the Brexit deal – especially over the Northern Ireland border.
Mr Barnier accused Mr Johnson of creating “two lost years” when it came to the EU-UK relationship by dragging the UK into a legal battle over the Northern Ireland protocol.
But he said the EU had changed and that the UK could return any time.
"The door is open any time. I always say that during the negotiation and after negotiation, the door remains open.
“To be clear, the UK government, the current government, the next government knows exactly what are the terms and the conditions to be part of the single market to be part of the customs union to join the EU," he said.
"The point is to know what would happen during this time. How far what would be the size of divergence between us and a new country. So this is a point."
Mr Barnier, a former French government minister, led negotiations over the Brexit divorce deal, or withdrawal agreement, for Brussels.
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