It’s not every day that the ambassador of one of Britain’s closest allies calls the prime minister a congenital liar. Sylvie Bermann, who was until 2017 France’s envoy at the court of St James, is amused by the fuss in London over her portrait of Boris Johnson as a clever but reckless opportunist with no regard for the truth. Johnson, she wrote in her new book Goodbye Britannia, “lies to embellish reality, lies as a game but also as an instrument of power”.

A shower of Brexiteer wrath has been lobbed back across the Channel. “I’ve come in for the usual stuff: ‘Without us, you’d still be under the Third Reich’ and so on,” Bermann says, talking in her flat on the Paris Left Bank. “I didn’t mean that to be provocative. It’s just a factual description. He built his career on lying. He was sacked from The Times because he lied. Everyone knows it.” She has also received congratulations from British opponents of Brexit.

She says she likes Johnson but the prime minister, with David Cameron, are the guilty parties in Goodbye Britannia, her testimony as a front-seat observer of what she, like the whole continental establishment, sees as a disastrous act of self-sabotage by a country that was doing rather well out of its EU membership.

Bermann, 67, a former ambassador to China and Russia and the first woman to reach the pinnacle of the French diplomatic service, casts an affectionate but incredulous eye over the bout of irrationality that afflicted a country that the world admires for its pragmatism.

Sylvie Bermann contends that no one saw Brexit coming, not even the people who voted Leave.

A veteran of the United Nations and Brussels, she admits that she had no great UK expertise when she took up her post in 2014 but had learned its ways as a young vice-consul in colonial Hong Kong in 1979-80, stays in England and contact with its skilled diplomats. She greatly admired the country of Jane Austen and Monty Python. “I loved British humour, with its lively intelligence that is not without self-mockery,” she writes.

She strikes a wistful note in describing her shock over the Brexit vote. The ambassador left London the year after the referendum to become ambassador to Moscow, her last post before compulsory retirement last year.

“When I arrived [in Britain] I had the impression of a country firmly attached to its modernity with a flourishing economy. I was struck by the charm and all the new buildings and the mixture of old England with the modern … I lived in New York in the Nineties but I felt London had become the new world city. More exciting than New York,” she says. French leaders including a young Emmanuel Macron looked to London for tips on success and quality of life.

“I didn’t mean that to be provocative. It’s just a factual description. He built his career on lying.”

Britain has always fascinated the French. “It has that side that is totally exotic. It needs explaining.”

Despite her close contacts with the British intellectual and artistic world as well as the politicians, the Brexit vote came as a shock. “No one saw it coming, including the Brexiteers who always told me, ‘It’s never going to happen. We’ll never have the courage to leave’. ” Johnson was one of those predicting a Remain vote, she says. “He was saying it’s not going to happen … That wasn’t his goal. I think his main aim was to position himself with the hope of replacing Cameron.”

After the vote Britain felt sour, she says. The anger and hostility that had been under the surface was suddenly legitimized. The Brexit vote was the first eruption of an alarming global trend of populist rejection of “elites” and their devotion to charismatic manipulators.

She sees Johnson as a cultivated version of Donald Trump and President Bolsonaro of Brazil. Johnson and his Brexiteers got away with a fantasy version of Britain and the EU, she writes. “By feeding them false anxiety-inducing information about the EU and immigrants, the populists cleverly manipulated the people.” Her words closely echo Macron’s thoughts on the matter.

Britain has changed, she told me. “You feel all these little things you didn’t feel before. The cliché was that the British were very open. Very indifferent about whether you belonged to one religion or another. We had the impression it was a country that was more open than ours, more free, more optimistic. It’s different now.”

Cameron stumbled into the disaster by failing to make any positive argument for EU membership, she says. “He was always saying we’ll be safer and stronger and better off without explaining why. The other side said, ‘Take back control’ and people got the impression that they would recover something.”

France offered to help to boost the argument but Cameron refused every time, she says.

A Remain vote would have swung Britain the other way, she believes. “The people who were racist and aggressive would have shut up. They were like that already but they were legitimized by the success of Brexit. And that’s what happened in the United States,” she says. “If Trump hadn’t been elected everything that appeared might not have happened. Like the gilets jaunes in France,” she added, speaking of the French anti-government grassroots revolt. “We didn’t see them coming because the signals were too weak. If Marine Le Pen [National Rally leader] is elected it would legitimate a form of xenophobia.”

She sees Johnson as a cultivated version of Donald Trump and President Bolsonaro of Brazil.

Bermann says Britain is a victim of its delusions about an imagined past and its belief that it won the war single-handedly, without the sacrifice of 22 million Russians and US might.

“The partisans of Brexit are reciting a history in which the UK is never defeated, never invaded. The corollary of an England saving Europe is a detestation of Germany and contempt for cowardice — the term is often used for those who allowed themselves to be occupied, not to mention collaborated,” she writes.

While Britain had among the finest universities in the world, the popular press, led by the Daily Mail, she says, helped the country to imagine it was living in a world of Dad’s Army and Downton Abbey. Its slogan, she suggests laughing, should be the Beatles’ lyric: “I believe in yesterday.”

A boy who won’t be good might just as well be made of wood.

With the world quickly resolving into three poles of economic and military power — the US, the EU and China — Britain has doomed itself to unappetizing choices, she says. By removing itself from a union that is a big player and by putting up the backs of its former partners Britain has welded together a continent that it has throughout history worked to divide. “London will have succeeded in bringing together a continental bloc of 27 countries. This was the famous blockade organized by Napoleon and which England so feared.”

China is the big global challenge, she says, and Britain has downgraded its influence. In the book, she asks: “How has this country, whose influence had been decisive in Brussels, which insolently rolled out the red carpet for French entrepreneurs and which Xi Jinping had elected in October 2015 as the gateway to Europe, undertaken to scuttle itself?”

Britain now faces few options. It can become an American state “dependent on Uncle Sam” or it can attach itself to the Union, enjoying none of the advantages of membership, she says. President Biden believes Brexit is a “historic error” and is not keen on close ties with Johnson’s Britain, she writes.

Its slogan, she suggests laughing, should be the Beatles’ lyric: “I believe in yesterday.”

“Joe Biden has a very negative view of Boris Johnson, who he calls a ‘physical and emotional clone of Donald Trump’.”

Bermann, who wrote a book on China after her years as ambassador, has been accused by some of indulging an undemocratic regime. She responds: “Democracies are in crisis. There was Brexit, Trump, the gilets jaunes. The Chinese have stopped listening to western criticism. They say, ‘We don’t need you any more’. I don’t think the Chinese want us to have their system. What they want is that we leave them in peace.”

Bermann, who has not been married and has no children, was born in the Jura mountain region, the daughter of lawyers. One of her grandmothers was Russian. She regrets that French rules forced her to give up diplomatic work. “I adored the job. Getting to know people. Writers, artists, the politicians.” She counts the writers Julian Barnes and Jonathan Coe as friends.

Brexit is already hurting the UK, she says. “Since January 1, it’s true they have been better than us over the vaccines [she had Covid-19 last March] but the rest is rather difficult. Amsterdam has overtaken London for stock trading. The fishermen are absolutely furious and say, ‘We’ve been lied to’.”

She is not too pessimistic about Britain, however. “The British are not people who cry over spilt milk. They will bounce back because they are dynamic while having the talent.”

Sylvie-Agnès Bermann

Born: October 19, 1953, in Salins-les-Bains, France

Education: History degree from Paris-Sorbonne University, graduate of the French Institute of Oriental Languages and Civilisations, the French Institute of Political Studies and of the Beijing Language Institute

Career: Started out at the French Ministry of Foreign Affairs in 1979, where she later headed its directorate for the UN. As ambassador to China between 2011 and 2014, she became the first woman to hold the post of French ambassador to a country that was a permanent member of the UN security council. French ambassador to the United Kingdom between 2014 and 2017, before becoming ambassador to the Russian Federation.

Her book, Goodbye Britannia, was published this year.

Charles Bremner is the Paris correspondent for The Times