Michel Barnier was often puzzled and just as frequently shocked by the antics of his British counterparts and the chaos of UK politics during the more than four years that he negotiated the Brexit withdrawal agreement on behalf of the EU.

A conscientious bureaucrat and — from the evidence of his Brexit diary — a relentlessly high-minded politician who has now returned to his native France to weigh his chances as a possible centre-right candidate in next year’s elections, he may even have felt a tinge of envy at the way that “baroque personality” Boris Johnson and successive British cabinet ministers breezily went back on their promises and played fast and loose with the facts for political gain.

“As ever, Boris Johnson is direct, perhaps a little too direct,” Barnier notes primly after meeting the prime minister at 10 Downing Street in January 2020 with European Commission president Ursula von der Leyen. “He has the assurance of someone who has won an election: ‘No more Brexit! Finito! People want to move on.’” The negotiations to strike an agreement on a future relationship were, of course, to drag on for the rest of the year, and in reality are continuing today, with disputes over everything from customs controls and the Northern Ireland protocol to fishing rights in the Channel.

The “Great Illusion” of Barnier’s title, published in French on Thursday and in English in the autumn, is a reference to the book of the same name written more than a century ago by Norman Angell, who said war in Europe had been made improbable by the economic ties between nations — and who was quickly proved wrong by the first world war. Barnier also prefaces the book with the lament of King Lear, who in anguish beats his own head “that let thy folly in/And thy dear judgment out!”.

In his account of what he calls the “tragicomedy” of Brexit, Barnier is scathing about the fundamental lack of seriousness on the British side when it came to implementing the momentous decision of the 2016 referendum, especially after Johnson replaced Theresa May as prime minister. He has some sympathy for May, though he is baffled by her decision to rule out all the best options for a “soft” Brexit, such as remaining in the customs union.

“For her, this is not really about negotiating with the European Union but a much more violent negotiation, almost hour by hour, with her own ministers and her own party,” he wrote in June 2018. “And meanwhile, the clock is ticking.”

Barnier has delivered a blow-by-blow account of the years of wrangling and interminable meetings in Brussels, London and other European capitals — during which he had two grandchildren and caught Covid-19 — but despite its subtitle it contains few secrets.

A weakness is that Barnier lacks the twinkling, observant eye of the successful diarist. The reader loses count of the number of times Barnier praises his hardworking staff for their diligence or is given a chaleureux (warm) greeting by those he meets.

One can see why Barnier’s friend and political colleague Jean-Pierre Raffarin, the former French prime minister, once said of him: “He’s no poet. He’s not a gambler, a player or an acrobat. But he is solid and reliable.”

And reliable he was through the long years of Brexit negotiations. Barnier rigorously consulted EU institutions and the leaders of its 27 remaining member states, and in one sense the diary is a catalogue of failed British attempts to split the EU and “cherry pick” the desired aspects of a future relationship without paying a price for leaving the world’s biggest single market.

“I can’t work out what’s stopped the UK up to now from being ‘Global Britain’ except its own lack of competitiveness,” Barnier writes. “Germany is ‘Global Germany’ while being solidly inside the EU and the eurozone.”

Buried in this book is the important idea that Johnson, Nigel Farage and the other nostalgics of Brexit Britain have too lightly dismissed the dangers of conflict and extremism that have been taken to heart by France, Germany and the rest of the EU since the second world war and the cold war that followed. Indeed, the first person to feature in the text — in a short preface entitled “A Warning” — is the French far-right leader Marine Le Pen, exulting over the result of the Brexit referendum.

Barnier, whose lack of charisma means commentators give him little chance of mounting a successful bid for the French presidency in 2022, almost revels in his own stolidity. When European Council president Donald Tusk mocks May with an Instagram photo of himself offering her a piece of cake with “no cherries”, he says it “reinforces my determination to avoid all forms of aggressiveness, emotion or passion” in confronting the British.

Serious to the end, and mindful of Johnson’s easy-going approach to international treaties as well as the differences that continue to sour EU-UK relations, Barnier insists that the “objective and precise” implementation of the Brexit agreements is as important as the negotiations that preceded them.

“Brexit is a failure for the European Union,” Barnier concludes on the day of the UK’s formal departure, January 31 2020. “It’s also a mess for the United Kingdom and for us.”

La Grande Illusion: Journal secret du Brexit (2016-2020), by Michel Barnier, Gallimard, RRP€23, 544 pages

Victor Mallet is the FT’s Paris bureau chief

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