In scathing comments, Mr Barnier also said Mr Johnson’s ministers “did not know” or understand the consequences of Brexit.
“From the very first day, the UK ministers not only underestimated the consequences of Brexit – they did not know the consequences of Brexit,” Mr Barnier told the Financial Times.
The ex-negotiator criticised then PM Theresa May for ruling out any membership to the single market or customs union from the outset, and said Mr Johnson made “a huge mistake” by imposing deadlines on Brexit. “He had no time.”
Mr Barnier revealed that he believed Ms May when she warned of a potential no-deal Brexit, but did not believe her successor as he tried to up the ante with threats to crash the country out of the EU without any deal.
“Johnson said once, ‘I want a deal because I need a deal.’ This sentence was key for me,” he said on the former Tory leader’s bluff – known as the “madman theory”, when a negotiator attempts to suggest he is capable of anything.
Mr Barnier added: “I wasn’t surprised by the madman strategy. I was told that this strategy was taught in university in the UK.”
The former negotiator also ridiculed the UK government’s attempts to “divide” EU countries in a bid to weaken the European Commission’s hand during talks.
“The British did not understand. They tried every week to divide us. I visited one capital every week: the day before I came, there was a British minister, and the day after. Which is incredible.”
The former French cabinet minister – who failed in his bid to become the French Republicans’ presidential candidate in 2022 – was enthusiastic about the possibility of a Labour government.
He said Sir Keir’s plan for a new UK-EU veterinary standards agreement to ease some of the costly trade checks on food and agricultural products “seems to be pragmatic and possible”.
But he played down the idea of rewriting the Brexit deal at the 2025 review of the EU-UK Trade and Cooperation Agreement. Referring to shadow foreign secretary David Lammy’s pledge to go through it “page by page”, Mr Barnier said: “Good luck.”
He told the FT that there is “no room” for reopening the deal, although “it’s always possible to improve the functioning of this agreement, on some technical points”.
Mr Barnier also made clear he was against the idea of reopening the Brexit deal to delay tariffs to be imposed on the UK and EU automobile industry from January 2024.
Manufacturers across the continent have warned of a “devastating price war” when the new rules of origin come into effect, threatening the emerging electric vehicle market because of China’s domination of electric battery production.
Rishi Sunak’s government is pushing the European Commission to agree to delay the costly new rules set to come in at the start of next year. But Brussels has shown no sign it is willing to budge.
Mr Barnier said he was not in favour of “any type of flexibility”, concerned that it could set a precedent to reopen more of the trade agreement. “[Britain] lost the financial passport: there will be no flexibility.”