Michel Barnier openly wondered whether Boris Johnson was pursuing a “madman strategy” in Brexit negotiations and came close to losing faith in the UK’s ability to keep its word during the gruelling talks, according to his diaries.

The EU’s chief negotiator for more than three years writes that the EU side was dumbfounded by the UK prime minister’s unpredictable approach — which Barnier refers to at one point as “political cinema”.

Barnier’s diaries, published in French on Thursday and in English later this year, are being released as the EU and UK grapple with implementing the post-Brexit deals they laboriously negotiated in the aftermath of Britain’s 2016 vote to leave the bloc.

The EU and UK have clashed since the start of the year on how to roll out a new system of trading rules for Northern Ireland, while France and Johnson’s government are currently locked in a dispute over fishing licences.

Barnier, who is now mooted as a potential centre-right candidate in next year’s French presidential election, stepped down from his role as chief Brexit negotiator after the UK’s exit from the EU single market on January 1.

His book — The Grand Illusion: A Secret Diary of Brexit — highlights one key moment of crisis in September 2020, when Johnson’s government moved to override its own divorce deal with Brussels in relation to arrangements for preventing a hard border on the island of Ireland.

Barnier writes on September 8 2020: “The team currently in 10 Downing St does not measure up to the stakes and challenges of Brexit . . . I simply no longer have [a feeling of] trust. Well, we need trust to conclude an agreement.”

Barnier speculated whether the British had simply sought to “give themselves another lever in the negotiations, ready to abandon it later. So, a ‘madman strategy’,” he writes.

The 542-page book records the EU’s astonishment at some of the UK prime minister’s suggestions during the two sides’ negotiations on a future-relationship agreement — a stop-start process during 2020 that was dogged by the Covid-19 pandemic and entrenched disagreements about trading conditions and fish.

Ahead of a dinner in December 2020, Barnier confided to his diary his impression that Britain’s prime minister was inadequately prepared compared with his EU counterparts.

During the meal in Brussels with European Commission president Ursula von der Leyen, and a previous encounter with her predecessor Jean-Claude Juncker, Barnier had the impression that Johnson “had not taken the time to go into the detail himself, with his teams” before the meetings.

At the December 2020 dinner this impression was underscored when Johnson floated the idea of striking a defence and foreign policy co-operation pact with the EU if a broader future-relationship agreement could not be found.

Barnier quickly pointed out to Johnson that this directly contradicted the UK’s stated position against including these two areas in the future-relationship talks. Barnier says Johnson replied by asking his own team: “Who gave that order?” The Frenchman goes on to note: “The theatrics continue.”

Barnier’s recollections repeatedly make reference to UK politicians’ failure to come to terms with the complex consequences of the 2016 referendum decision. For example he recalls being “stupefied” by Theresa May’s Lancaster House speech in 2017, and in particular at the “number of doors she shut, one after the other” when it came to the scope of the UK’s future relations with the EU.

Brussels’ bafflement at the UK approach to Brexit increased after Johnson came to power in 2019.

At one point, the EU’s chief negotiator sought guidance from a column penned years earlier by Johnson in The Telegraph. In the article, published in 2013, Britain’s prime minister advocated “throwing a dead cat on the table” to escape from an argument one is losing.

When Johnson decided to suspend talks in mid-October 2020, after taking umbrage at a statement adopted by EU leaders, Barnier describes it as a “psychodrama orchestrated by London”.

The diaries make clear that Barnier sees Johnson as an embodiment of Britain’s struggle to come to terms with the realities of Brexit.

He cites a meeting between Johnson and Juncker in a Luxembourg restaurant in 2019 where the commission laid out its position on arrangements for Northern Ireland.

Barnier recalls thinking that Johnson’s eyes were being opened to a “series of technical or legal problems that had not been sufficiently clearly explained to him by his own team”.

Number 10 declined to comment on Barnier’s book.