The British and French leaders had their own private discussion later at the summit, not least about easing customs checks on goods travelling from Britain into Northern Ireland.

What followed left the Elysee Palace infuriated. A UK figure familiar with the chat briefed some reporters that Mr Macron had misunderstood that Northern Ireland was a full part of the UK.

“The Prime Minister said to Mr Macron ‘How would you like it if the French courts stopped you moving Toulouse sausages to Paris?’” the source recounted to a handful of publications, including The Telegraph.

“He replied that it was not a good comparison because Paris and Toulouse are both part of the same country.”

The briefing hit the front pages and triggered a rebuttal from Mr Macron’s team, who argued that he had been noting the differences in the examples, not least the sea separating the mainland UK from Northern Ireland.

“Let’s not lose time in disagreements created in corridors or antechambers,” Mr Macron said when asked about the claim. Translation: That allegation was cooked up by the Britons.

That sense of ill feeling, that Mr Johnson and his team do not play it straight, permeates Mr Macron’s team, according to French reports, and helps explain recent deteriorations.

Little did the French president know at the time, but a second source of fierce disagreement was being developed in a room away from the spotlight during the Cornish gathering.

Awkwardness over Aukus 

Joe Biden, the US president, and Scott Morrison, the Australian prime minister, met with Mr Johnson for three-way talks about which the press were given little information at the time.

We now know one of the agenda items on the table: a new defence pact that would see Washington and Westminster help Canberra acquire nuclear-powered submarines.

When the deal eventually became public in September, Mr Macron was left infuriated because it replaced the €56 billion (£47 billion) “contract of the century” submarine deal France had signed with Australia in 2016.

The diplomatic reaction was swift and heated. Jean-Yves Le Drian, the French foreign minister, called the pact a “stab in the back”. Paris recalled its ambassadors from Washington and Canberra – though, tellingly, not from London.

“With Britain, there is no need,” Mr Le Drian said by way of explanation. “We know their constant opportunism, so there is no need to bring our ambassador back to explain.”