Queen Elizabeth has been caught on camera saying Chinese officials were "very rude" to the British ambassador during a state visit to Britain by President Xi Jinping last year.
She made the comments at a garden party at Buckingham Palace on Tuesday, the same day that Britain's prime minister, David Cameron, was filmed making undiplomatic remarks about Nigeria and Afghanistan being corrupt countries.
His comments were especially awkward as he is hosting both countries' leaders at an anti-corruption conference in London on Thursday, while the queen's comments are not helpful to the British government's determined efforts to court Beijing to boost trade ties with China.
Under her constitutional role, the 90-year-old monarch never makes any politically or diplomatically sensitive comments in public, and it is rare for the content of her private conversations to be revealed.
In footage broadcast by the BBC, the queen is seen meeting senior police officer Lucy D'Orsi, who is introduced by an official as having been in charge of security during Xi's visit in October last year.
"Oh, bad luck," the queen says in response.
D'Orsi then describes her dealings with Chinese officials as "quite a testing time" and recounts that at one point they had walked out of a meeting and told her "the trip was off".
The queen says: "They were very rude to the ambassador."
The BBC reported that in China, items about the queen's remarks were censored from its news bulletins.
The Chinese authorities often censor items they object to from foreign news bulletins, which can only be seen by very few people in China as foreign TV channels are only allowed in high-end hotels and a tiny number of select apartment buildings.
A spokeswoman for the queen said: "We do not comment on the queen's private conversations. However, the Chinese state visit was extremely successful and all parties worked closely to ensure it proceeded smoothly."
Xi's visit was full of pomp and ceremony, with Cameron and his finance minister George Osborne keen to impress the Chinese leader and present Britain as China's firmest friend in Europe.
The queen has been careful to keep her views to herself during her 64-year reign, but several other members of Britain's royal family have made undiplomatic comments about China in the past.
The queen's husband, Prince Philip, warned some British students in China in the 1980s that they would get "slitty eyes" if they stayed there too long.
Her eldest son, heir-to-the-throne Prince Charles, has skipped two state banquets for Chinese guests in Britain, and described some Chinese officials in a journal that was leaked to the media as "appalling old waxworks".