Speaking at a Centre for European Reform conference, Barnier said: "On financial services, UK voices suggest that Brexit does not mean Brexit. Brexit means Brexit. Everywhere."
He added: "They say there will be no changes in market access for UK established firms, the say joint UK-EU rules would be decided in a new, symmetrical process... between the EU and the UK and outside of the jurisdictions of the European Court of Justice.
"This would contradict the April European Council guidelines, which are my mandate, which stress the autonomy of the European decision-making, the integrity of our legal order, and of the Single Market.
"The legal consequence of Brexit, the legal consequence, is that the UK financial service providers lose their EU passport."
However, he said there was a possibility that the EU could judge some of the UK rules as equivalent to its own, and said a good deal would facilitate as much market access as possible, while seeking to avoid a situation whereby the UK and the EU trade under WTO rules.
Barnier hammered home points he made earlier this year when he spoke to a select committee of the House of Lords about how the UK should approach trade negotiations.
Whether the UK decides to remain aligned with EU regulations, or move away from them, will be "decisive" in how the EU responds to trade negotiations, he said, suggesting the national parliaments of the EU27 would refused to ratify a deal if the UK wishes to ditch EU standards for competitive advantage.
"I don't say that to create a problem, I say that to avoid a problem," Barnier said. "I have to say clearly what are the conditions for the success of the process of ratification in the second stage of the negotiations, about the future relations."