The Queen has been caught on camera describing Chinese officials as "very rude".

The monarch was overheard making the uncharacteristically unguarded comments during a discussion with a Metropolitan Police Service commander who described difficulties organising the state visit of Chinese premier Xi Jinping in October.

The Queen quipped it had been "bad luck" when she heard how Commander Lucy D'Orsi had been assigned to be Gold Commander of the police operation around the visit of the world leader.

Palace officials have declined to comment on the private conversation with the police officer during an event marking the Queen's 90th birthday, but stressed president Xi's visit had been "extremely successful" and all parties had "worked closely to ensure it proceeded smoothly."

The comments were captured by an official Palace cameraman soon after the Prime Minister was filmed describing Afghanistan and Nigeria to the Queen as "fantastically corrupt" in a separate diplomatic mishap.

When the Queen's comments were reported on BBC World, the broadcast was blacked out in China.

Footage showed the Queen being introduced to Ms D'Orsi by an official. When the official told how Ms D'Orsi had been assigned Gold Commander for the operation, the monarch quipped: "Oh, bad luck."

The official continued to tell the Queen the officer had been "seriously undermined by the Chinese" in their handling of the visit, but added the officer had managed to "hold her own".

As Ms D'Orsi asked if she knew it had been a "testing time", the Queen interjected: "I did."

The officer described how Chinese officials walked out of a meeting with Barbara Woodward, British ambassador to China, at Lancaster House, telling them the trip was off.

The Queen said: "They were very rude to the ambassador."

The remarks were recorded as the Queen greeted guests in the gardens of Buckingham Palace for an event marking her 90th birthday.

A palace spokeswoman 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."

The Metropolitan Police said it would not comment on the private conversation.

It is not the first time a member of the British royal family has made undiplomatic comments about Chinese officials.

The Prince of Wales branded Chinese diplomats "appalling old waxworks" in a private journal entry about the Hong Kong handover ceremony in 1997.

Earlier on Tuesday, David Cameron was filmed describing Afghanistan and Nigeria in unflattering terms while he discussed an upcoming anti-corruption summit with the Queen and the Archbishop of Canterbury.

"We've got some leaders of some fantastically corrupt countries coming to Britain," Mr Cameron told the Queen. "Nigeria and Afghanistan - possibly the two most corrupt countries in the world."

Afghanistan's Ashraf Ghani and Nigeria's Muhammadu Buhari are due to attend the London summit and each has written an essay for an accompanying book, discussing their efforts to tackle graft in their countries.

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