The Queen has been caught on camera saying Chinese officials were "very rude" during a state visit to Britain last year by President Xi Jinping.
She made the comments to a senior police officer at a garden party at Buckingham Palace this week, the same day that UK Prime Minister David Cameron was filmed making undiplomatic remarks to her about corruption in Nigeria and Afghanistan.
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.
The Queen's remarks may not be helpful to the British government's determined efforts to boost trade ties with China.
The invitation to President Xi in October last year was part of the government's policy of courting Chinese investment, and at the time London said the visit would herald a "golden era" in relations with Beijing.
The BBC has reported that in China, items about the queen's remarks were censored from its news bulletins.
The Queen's remarks were filmed as she was introduced to Metropolitan Police Commander Lucy D'Orsi, who the monarch was told had overseen security during President Xi's visit to the UK last year.
She is heard to respond: "Oh, bad luck."
An official went on to tell the Queen that Commander D'Orsi had been "seriously, seriously undermined by the Chinese, but she managed to hold her own and remain in command".
Ms D'Orsi told the Queen: "I was the Gold Commander so I'm not sure whether you knew, but it was quite a testing time for..."
"I did," the Queen said.
Ms D'Orsi continued: "It was at the point they walked out of Lancaster House and told me that the trip was off, that I felt..."
The Queen said: "They were very rude to the ambassador."
Ms D'Orsi replied: "They were... it was very rude and undiplomatic I thought."
The Queen described it as "extraordinary".
A Buckingham Palace spokesman later said they 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."
Royal garden parties are filmed by the cameraman who covers the palace for UK broadcasters.
Foreign Secretary Philip Hammond said that "at times it got a bit stressful on both sides" but that state visits were big logistical challenges.
At the palace, Ms D'Orsi told reporters it had been "rewarding" to be thanked by the Queen for her work during the state visit.
She said the Queen and her own mother had chatted about the benefits of being grandmothers.
A Metropolitan Police spokesman has said it was not prepared to discuss a private conversation.
Buckingham Palace and Chinese officials both said the Chinese visit had been "successful".
Coverage of the comments has been censored in China where a report on BBC World News was blanked out.
Instead, state media outlets have dedicated their coverage to the Queen's dress sense and notable party attendees.
Social media users have been keen to comment, but many appear to have had their posts removed by online censors.
Some managed to bypass filters by using English rather than Chinese to repeat the Queen's comments.