The Queen has been filmed saying Chinese officials were "very rude" during last year's state visit by President Xi Jinping.
She was discussing their treatment of Britain's ambassador to China with a senior police officer at a Buckingham Palace garden party on Tuesday.
Buckingham Palace said the Chinese visit had been "extremely successful".
Chinese officials in both London and Beijing also recalled the visit's "success".
The invitation to President Xi was part of the government's policy of courting Chinese investment.
The Queen's remarks were filmed as she was introduced to Metropolitan Police Commander Lucy D'Orsi, who the monarch is told had overseen security during President Xi's visit to the UK in October.
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".
Commander 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.
Commander 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."
Commander 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: "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."
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.
In Beijing, a government spokesperson described the trip as very successful, starting a "Golden Era" of relations.
Asked several times if that era continued today he neither confirmed nor denied it.
The Chinese Embassy in London said both sides at "the working level" had made great efforts towards the visit's success.
At the time of the visit, the Queen hailed it as a "milestone" and declared Anglo-Chinese ties were being taken to "ambitious" new heights.
President Xi Jinping and his wife Peng Liyuan were honoured with a state banquet at Buckingham Palace, hosted by the Queen.
By Peter Hunt, BBC royal correspondent
Last year, the official talk was of a trade focused state visit ushering in a "golden time" for relations between the two countries.
We now know, thanks to a conversation in the Queen's palace garden, that it was a testing time behind the scenes.
Blunt talking, in public, is normally the preserve of the Queen's husband.
In the 80s, Prince Philip warned some British students in China that they'd get "slitty eyes" if they stayed there too long.
And Prince Charles - who's avoided two Chinese state banquets in the UK - described some officials in a leaked journal as "appalling old waxworks".
Buckingham Palace - while not commenting on what they call a private conversation - have stressed all parties worked closely to ensure an extremely successful Chinese state visit proceeded smoothly.
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.
The Queen has largely avoided making political statements in her 64-year reign but it is not the first time her comments on controversial areas have been reported.
In the build-up to Scotland's 2014 referendum on independence, Buckingham Palace denied suggestions that the Queen would wish to influence the result, following reports that she was concerned.
Shortly before the vote she was heard saying she hoped people would "think very carefully about the future".
And in March this year, there were claims by the Sun newspaper that the Queen told former Deputy Prime Minister Nick Clegg in 2011 she was in favour of the UK leaving the European Union.
Buckingham Palace said it would not comment on "spurious" claims and complained to the press watchdog, while Mr Clegg called the story "nonsense".
Earlier on Tuesday, the prime minister was filmed at a Buckingham Palace event to mark the Queen's 90th birthday also making unguarded comments.
Talking about this week's anti-corruption summit in London, he said: "We've got some leaders of some fantastically corrupt countries coming to Britain. Nigeria and Afghanistan, possibly the two most corrupt countries in the world."
The Archbishop of Canterbury Justin Welby intervened to say: "But this particular president is not corrupt. He's trying very hard." before Speaker John Bercow said: "They are coming at their own expense, one assumes?"
BBC political correspondent James Landale said the prime minister's remarks were outspoken, unguarded and ostensibly embarrassing, but they were not untrue.
In Transparency International's 2015 corruption perception index, Afghanistan was ranked at 167, ahead of only Somalia and North Korea, Nigeria was at 136.
With his remark, the archbishop was believed to have been referring to Nigeria's President Muhammadu Buhari, who won elections last year promising to fight widespread corruption.
Mr Buhari said he was "shocked" by the prime minister's comments, while a senior Afghan official said the characterisation was "unfair".
No 10 said the presidents of Nigeria and Afghanistan had "acknowledged the scale of the corruption challenge they face in their countries".