The usually cool-headed Frenchman showed flashes of anger as he was asked what concessions he had been prepared to make to his British counterpart David Davis during a frantic opening day of talks.
In response Mr Barnier, who is the EU’s chief negotiator, launched into a furious tirade against the Brexit decision but also insisted that the rest of Europe had no desire to “punish” the UK for choosing to quit.
His outburst came at the end of an evening press conference which began in more jovial fashion, with the pair announcing great progress had been made on the structuring of the talks and citizens’ rights.
Mr Davis in particualr was keen to portray the first salvo of the talks as highly constructive after earlier telling reporters that Britain and the EU had “more that unites us than divides us”.
But the Brexit secretary, who faced a volley of tough questions about the political chaos engulfing the Government back at home, was introduced in a brutal way to Mr Barnier’s steely resolve not to budge from his negotiating mandate handed down by EU leaders.
Asked what concessions he was prepared to grant the UK, the Frenchman raged: “The UK has decided to leave the EU, it’s not the other way around. The UK is going to leave the EU, the Single Market and the Customs Union, not the other way around.
“So we each have to assume our responsibility and the consequences of our decision and the consequences are substantial. It’s not about punishment, it’s not about revenge.
“Basically we are implementing the decision taken by the UK to leave the EU and unravel 43 years of patiently built relations. I will do all I can to put emotion to one side and stick to the facts, the figures and the legal basis. “
Moments before Mr Barnier had expressed his determination to reach a “fair deal” with the UK and once more laid into claims by Theresa May that the British side could walk away from the talks.
He told reporters: “A fair deal is possible and far better than no deal. That is…