Boris Johnson is reported to have called the French "turds" when he was foreign secretary, in a Brexit outburst filmed for a BBC documentary but never broadcast.
The Foreign Office persuaded the BBC to cut it from the film because it was "potentially awkward" for Anglo-French relations, according to a report in the Daily Mail.
It is reported that Mr Johnson made the remark in frustration at what he saw as French intransigence in Brexit negotiations as Theresa May was battling to secure a deal.
Pressed over the alleged insult during a leadership hustings in Exeter, Mr Johnson said: "I have no recollection of this comments."
When it was suggested that the comment was leaked by Jeremy Hunt's Foreign Office, Mr Johnson responded in French saying: "Bien je jamais" or "Well I never..."
The Mail quoted a leaked Foreign Office memo, dated 13 November last year, stating: "We negotiated the removal of one potentially awkward moment where the former foreign secretary calls the French 'turds' so as not to distract from the rest of the programme."
As a result, the remark was cut from the documentary, a "fly on the wall" three-part series called Inside The Foreign Office, filmed from summer 2017 until the summer of 2018 when Mr Johnson was foreign secretary.
Responding to the Mail's report, a BBC spokesperson told Sky News: "The programme set out to reflect the realities of life inside the Foreign Office.
"The production team made judgements about what was in the programme and they are satisfied that the programme achieves its ambitions and has the content they wanted."
By the time the film was broadcast in November last year Mr Johnson had resigned as foreign secretary, having quit in July 2018 in protest at the prime minister's Brexit agreement.
In the days running up to the screening of the film, with Mrs May still trying to win concessions from the French and the rest of the EU, the Foreign Office "panicked" when it was told the BBC planned to broadcast Mr Johnson's comment, the Mail reports.
The paper says diplomats begged the corporation to censor it, leading to "bizarre secret negotiations" over the issue. Diplomats were also worried about another attack by Mr Johnson on the French in the film, in which he accused them of "shafting Britain".
According to the Mail, the BBC initially stood firm, telling the Foreign Office it planned to broadcast both remarks on the grounds that it was an accurate and fair portrayal of Mr Johnson as foreign secretary.
The BBC said it had been guaranteed full editorial control over the programme, made up of hundreds of hours of behind the scenes filming at the Foreign Office over the course of a year, the paper reports.
The Foreign Office is said to have countered that Mr Johnson's comment could cause "significant damage" to Britain and made a mockery of the government's aim in agreeing to the documentary, which was to "promote Global Britain to a UK audience".
In a compromise, the BBC agreed to remove Mr Johnson's "French turds" remark but insisted it would retain his "shafted" comment, the Mail reports.
The latest controversy to hit Mr Johnson during the leadership campaign comes as his rival Jeremy Hunt again challenges him over his refusal to debate with him on Sky News.
In a letter to Mr Johnson about his Brexit claims, Mr Hunt writes: "I had hoped we would be able to debate these issues in person at Sky on Tuesday evening or next Monday, or at one of the proposed debates next week.
"But as you don't wish to face me in person before members have started voting, for now we must use your preferred method of debate, exchanging letters.
"In response to your questions, I believe with a trusted negotiator and the right team we should get a deal by 31 October. If however, there was no prospect of delivering Brexit with a better deal I would leave without one.
"I also believe the biggest risk to Brexit is a general election, and a Jeremy Corbyn government, before it is delivered."
Mr Johnson's latest policy announcement in the campaign is a pledge to "save the great British high street" by helping retailers deal with the rise of online shopping.
His proposals include:
* Planning reforms to allow much easier change of use on the High Street;
* Ending business rates on free-to-use cash machines in town centres.
* Making immediate use of the Â£675m Future High Street Fund to support and fund local areas' plans.
"High Streets are a vital part of British life," said Mr Johnson. "But they are coming under increasing pressure from the rise of online shopping - especially in the more rural areas.
"We need a bold vision to rejuvenate our high streets and to make sure that they remain places where people want to go, meet and spend their money.
"When we leave the European Union on 31 October we can make these bold changes. The high street is the heart of many towns across the country, a place for people to come together and support local business. This is an extremely exciting opportunity to revive our communities and we should grab it with both hands."
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