(CNN) So much for the "golden era" of UK-China relations.
The Queen has been caught on camera saying Chinese officials were "very rude" during President Xi Jinping's first state visit to the United Kingdom last year.
This rare moment of candor from the usually solidly diplomatic monarch was caught on camera by Buckingham Palace's own staff, as the Queen greeted a senior Metropolitan police officer.
"Oh, bad luck," she says, after hearing commander Lucy D'Orsi had been in charge of security for Xi's visit.
"I'm not sure whether you knew, but it was quite a testing time for me," D'Orsi tells the Queen.
"Yes, I did," she responds.
"They were very rude to the ambassador," the Queen adds, referring to Barbara Woodward, Britain's first female ambassador to China.
The Metropolitan Police said it does not comment on private conversations of officers.
A spokesman for Buckingham Palace said in a statement: "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."
Asked about the Queen's comments at a regular press conference, Chinese Foreign Ministry spokesman Lu Kang said "the visit launched a golden era in China-UK bilateral ties."
He added that he had "no knowledge" of any threats to call off the visit, referenced by D'Orsi in her comments to the Queen.
Chinese President Xi Jinping visits UK
Chinese President Xi Jinping addresses an audience of dignitaries, including British Prime Minister David Cameron, at the Manchester airport in Manchester, England, on Friday, October 23. It was the end of a four-day state visit for Xi, who traveled to the United Kingdom with his wife, Peng Liyuan.
Xi tours the National Graphene Institute at the University of Manchester on October 23.
Manchester City soccer player Sergio Aguero takes a selfie with Xi and Cameron during a visit to the soccer club's practice session on October 23.
Xi and Cameron view the ball used in the first World Cup final during a visit to the City Football Academy in Manchester on October 23.
Cameron drinks a pint of beer with Xi at a pub in Princes Risborough, England, on Thursday, October 22.
Cameron and Xi hold talks with their aides at Chequers, the Prime Minister's official country residence, on October 22.
Xi sits with Britain's Prince Andrew, left, at the opening of the UK Confucius Institutes and Confucius Classrooms Annual Conference in London on October 22.
Xi views satellite coverage screens in the Network Operation Centre control room during a visit to Inmarsat, a satellite telecommunications company, in London on October 22.
Xi signs the distinguished visitors book before a banquet at the Guildhall in London on Wednesday, October 21.
Xi makes a speech during the banquet.
Xi makes a speech during a banquet in London on October 21, resulting in nearby attendees -- including Prince Andrew -- struggling to stay awake.
Xi wears 3-D glasses as he views robotic equipment October 21 at Imperial College London.
Xi and Peng inspect equipment at Imperial College's Hamlyn Centre for Medical Robotics.
Peng receives a gift from professor Yike Guo during the visit to Imperial College.
Xi chats with actor Jackie Chan, second left, and Catherine, Duchess of Cambridge, at Lancaster House in London on October 21.
Xi was at Lancaster House to attend a presentation by the British Academy of Film and Television Arts.
Cameron greets Xi as he arrives at 10 Downing Street on October 21.
Xi and the Duchess of Cambridge raise their glasses at a state banquet at Buckingham Palace on Tuesday, October 20.
The Chinese leader and first lady join Queen Elizabeth II and Prince Philip at a state banquet on October 20.
Prince Philip, Queen Elizabeth II, Xi and Peng view a display of Chinese items from the Royal Collection at Buckingham Palace.
Xi addresses lawmakers and peers in Parliament's Royal Gallery on October 20.
Xi walks with Prince Philip to inspect a guard of honor in London on October 20.
Xi shakes hands with Queen Elizabeth II during his welcome ceremony in London.
The Queen and Peng watch Xi inspect the guard of honor.
Household Cavalry stand at attention ahead of the welcome ceremony.
Pro-Tibet protesters hold placards and chant slogans as they demonstrate on Parliament Square in London on October 20.
Prince Charles talks with Xi at a London hotel on October 20.
China supporters pose for a picture in London on October 20.
British Foreign Secretary Philip Hammond sits with Xi at Heathrow Airport after Xi's arrival on Monday, October 19.
Hammond greets Peng after she arrived with her husband at Heathrow Airport.
Xi and Peng arrive at the airport.
Two gaffes in one day
The Queen's remarks were made on the same day as UK Prime Minister David Cameron was caught on camera telling the monarch that Nigeria and Afghanistan were "fantastically corrupt."
Cameron, speaking about an upcoming anti-corruption summit, said they were "possibly the two most corrupt countries in the world."
His remarks caused consternation online, and sparked an angry rebuke from Nigeria's presidential spokesman, who said they were "embarrassing."
Out of character
Chinese President Xi Jinping and the Queen during Xi's UK visit.
While Cameron has had his fair share of gaffes in the past, the Queen -- who recently celebrated her 90th birthday -- is usually careful to stay out of politics.
At the time, she hailed Xi's trip -- during which she hosted the Chinese leader and his wife Peng Liyuan at Buckingham Palace -- as a "milestone" visit.
Other members of the royal family have been less circumspect.
In the 1980s, Prince Philip told British students in China that they'd get "slitty eyes" if they stayed in the country too long.
Prince Charles is a personal supporter of the Dalai Lama, who has described the two of them as the "best of friends," and skipped a state dinner with Xi, though he met the Chinese leader at another engagement during his trip. China regards the Dalai Lama as a "separatist" and regularly pressures foreign governments not to have dealings with him.
In a leaked journal, Charles also described Xi's predecessors as "appalling old waxworks."
Golden era?
The Queen's remarks come during a supposed "golden era" of UK-China relations.
During a trip to a pub with Cameron, Xi toasted the country's stronger bond and said he was "deeply impressed by the vitality of China-UK relations."
Since then however, cracks have started to emerge, over China's alleged abduction of a British bookseller from Hong Kong, and Beijing's aggressive moves in the South China Sea.
In an opinion piece published in British media last week, China's ambassador to the UK, Liu Xiaoming, accused London of "ramping up" the issue and "meddling" in the region.
CNN's Steven Jiang, Max Foster and Damien Ward contributed reporting.