President Emmanuel Macron on Friday took issue with UK leadership front-runner Liz Truss declaring "the jury was out" on regarding France as an ally of Britain.
Mr Macron issued a warning to London not to "lose its bearings" as the Conservative Party picks a successor to Prime Minister Boris Johnson.
The French leader spoke of serious problems if the UK was ready to define France as foe. "The United Kingdom is a friend of France," he said on Friday on a visit to Algeria. "It's never good to lose your bearings too much in life.
"The British people, the nation that is the United Kingdom, is a friendly, strong and allied nation, whoever their leaders are — sometimes despite their leaders or the little mistakes they may make in statements on the stage.
"If we are not able, between French and British, to say we are friend or enemy — the term is not neutral — we are going towards serious problems," said Mr Macron, who appeared shocked and drew a sharp intake of breath when told of the remark after a visit to the Saint-Eugene cemetery in Algiers.
Ms Truss may have jeopardised diplomatic relations with France after her remarks on Mr Macron as an ally of the UK on the world stage, at a Conservative Party leadership hustings in Norwich on Thursday.
The host had asked her: "President Macron, friend or foe"?
"The jury's out," she responded to loud applause. "But if I become prime minister, I would judge him on deeds, not words."
A number of issues have affected the relationship between the UK and France in recent months, including migrant boat crossings in the Channel and travel chaos around Dover, which Ms Truss blamed on a lack of staffing by the French authorities.
Both the Foreign Secretary and her rival candidate Rishi Sunak were asked a series of quick-fire questions at the Norwich hustings. Mr Sunak had quickly answered "friend" when asked the same question.
The opposition Labour Party said Ms Truss's comment showed a "terrible and worrying lack of judgment".
Former Conservative minister Gavin Barwell also questioned the remark, tweeting: "You would have thought the Foreign Secretary was aware we are in a military alliance with France."
The German ambassador to the UK also said solid relations with France were of "crucial importance".
Miguel Berger, appearing on BBC Radio 4's Today programme, said: "I would say that the relationship with France is of crucial importance for the United Kingdom, so my recommendation would be really to look for a relationship that is as close as possible."
Pressed on whether Ms Truss's comments were wise, he said: "The relationship with France should be as close as possible.
"I think there needs to be an effort to reach a good understanding and co-operation with our French neighbours."
Elsewhere during the hustings, Ms Truss conceded that if it were a choice between relying on France or China for nuclear expertise, she would pick France.
Taking questions in front of an audience of Tory members, she said: "I'm very clear that we need to boost our nuclear industry, including Sizewell, including the small modular reactors that are produced in Derbyshire.
"Frankly, I would rather that we do have more home-grown nuclear expertise, and regrettably we lost that because we failed to do these things 20 years ago, or 30 years ago.
"If it's a choice between relying on France and relying on China, I would take France."
It comes after Ms Truss distanced the UK from the prospect of being part of a wider European political community after a meeting between Mr Johnson and the French president in June.
The Elysee Palace insisted that the Prime Minister had expressed interest in the idea, which could see non-EU states such as the UK involved.
Ms Truss denied the UK had ever been on board with such a proposal, saying afterwards: "That is not true.
"I don't know the exact words that President Macron has used but we have not agreed to that."
Asked whether she bought into "his political and economic community", she replied: "No."
She said last month that delays to holidaymakers' journeys near Dover were the fault of French authorities and had been "entirely avoidable".
However, a French politician blamed Brexit for the chaos.
Pierre-Henri Dumont, Republican MP for Calais, said the problems at the Kent port would reoccur, telling BBC News: "This is an aftermath of Brexit. We have to run more checks than before."