Boris Johnson was left infuriated on Saturday when Emmanuel Macron suggested in head-to-head G7 summit talks that Northern Ireland was not part of the UK.
The Prime Minister attempted to explain his frustration with the Northern Ireland Protocol by asking Mr Macron what he would do if sausages from Toulouse could not be moved to Paris, according to a UK government source.
The French president was said to have retorted by arguing that the comparison did not work because Paris and Toulouse were both part of the same country, incorrectly suggesting Northern Ireland is not within the UK.
Mr Johnson expressed bemusement at the remark to aides straight after the meeting and later hinted at the comment when telling Sky News that some leaders needed to get it “into their heads” that the UK was a “single country”.
The alleged blunder emerged after early morning talks in Cornwall, which Mr Johnson had first with Mr Macron, then Angela Merkel, the German Chancellor, followed by Ursula von der Leyen, the European Commission President, and Charles Michel, the European Council President.
Far from triggering a breakthrough in the stand-off over customs checks in Northern Ireland, the bilateral meetings triggered a wave of critical media briefings from all sides.
A source close to Mr Macron said he told Mr Johnson that there “needed to be a reset of the Franco-British relationship” but only if “he keeps his word with the Europeans”.
Ms Merkel is understood to have said she was open to finding “pragmatic” solutions to the Northern Ireland situation but that the protocol itself could not be renegotiated.
Ms Von der Leyen and Mr Michel told Mr Johnson the “rhetoric needs to be toned down” over Northern Ireland and tweeted: “Both sides must implement what we agreed on. There is complete EU unity on this.”
The Northern Ireland Protocol imposes customs checks on goods travelling from Britain into Northern Ireland in order to avoid a hard border on the island of Ireland.
Mr Johnson is now considering triggering Article 16 of the protocol, unilaterally reneging on elements of the deal, within weeks.
Next month customs checks on sausages and other chilled meats entering Northern Ireland from Britain are due to start under the terms of the Northern Ireland Protocol.
The UK is threatening not to bring them in, with Downing Street concerned checks on that and a range of other products risks undermining the integrity of the UK and inflaming tensions in Northern Ireland.
A UK government source said of the meeting: "The Prime Minister said to Mr Macron ‘How would you like it if the French courts stopped you moving Toulouse sausages to Paris?’
“He replied that it was not a good comparison because Paris and Toulouse are both part of the same country.
"The PM replied: ‘Northern Ireland and Britain are part of the same country as well’. He was pretty struck by it as quite revealing as to how they see the issue.”