The Queen has been caught on camera 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 Wednesday.
The invitation to President Xi was part of the government's policy of courting Chinese investment.
Buckingham Palace said the Chinese state visit was "extremely successful".
It came after David Cameron was overheard earlier in the day commenting that Nigeria and Afghanistan were "fantastically corrupt".
The Queen's remarks were caught on tape on Tuesday 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 say: "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".
The Queen asked: "They were very rude to the ambassador weren't they?"
Commander D'Orsi replied: "They were", and described a "very rude and very undiplomatic" incident when Chinese officials walked out of a meeting, which the Queen said was "extraordinary".
A Buckingham Palace spokesperson 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."
At the time, the Queen hailed it as a "milestone" visit 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.
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 that all parties worked closely to ensure an extremely successful Chinese state visit proceeded smoothly.
Although the Queen has largely avoided making political statements in her 64-year reign, 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 about the prospect of Scottish independence.
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."
After Mr Cameron's comments, 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".
Meanwhile, Labour said a Tory government "hosting an anti-corruption summit is like putting the fox in charge of the chicken coop".
"The government is refusing to take meaningful action to close Britain's constellation of tax havens, which together constitute the largest financial secrecy network in the world," said shadow international development secretary Diane Abbott.
No 10 said the presidents of Nigeria and Afghanistan had "acknowledged the scale of the corruption challenge they face in their countries".