David Cameron has called Nigeria and Afghanistan "possibly the two most corrupt countries in the world".
The British Prime Minister was caught making the comments as he chatted with the British queen at an event to mark her 90th birthday.
In an apparent reference to the anti-corruption conference he is hosting in London later this week, he can be heard telling the Queen: "We've got the leaders of some fantastically corrupt countries coming to Britain."
Footage published by ITV News of the Buckingham Palace event - where the PM is seen chatting in a group including the Archbishop of Canterbury and Commons Speaker John Bercow - was shared on Twitter by ITV News deputy political editor Chris Ship.
Number 10 declined to comment directly on the premier's conversations with the queen but pointed out that the leaders of both countries had acknowledged the scale of the problem they faced.
Afghanistan's Ashraf Ghani and Nigeria's Muhammadu Buhari have written essays for a book accompanying the summit.
Mr Ghani, they said, acknowledges in his piece that Afghanistan is "one of the most corrupt countries on earth" and Mr Buhari that corruption became a "way of life" in his country under "supposedly accountable democratic governments".
Mr Cameron - who is stood next to Commons leader Chris Grayling in the group - said that the summit had been discussed at a "very successful" Cabinet meeting earlier.
The Archbishop - The Most Rev Justin Welby - is heard to intervene to make clear that "this particular president" is not himself corrupt.
Anti-corruption movement Transparency International ranked Afghanistan as 166th and Nigeria 136th out of 168 countries and territories in its Corruption Perceptions Index for 2015.
Mr Bercow is also heard making a joke about the summit, quipping: "They are coming at their own expense, one assumes?"
After some laughter, Mr Cameron answers: "Yes ... because it's an anti-corruption summit, everything has to be open, so there are no sort of closed-door sessions, it's all in front of the press.
"It's going to be ... it could be quite interesting. But anyway..."
The exchanges at the palace began with David Cameron joking about him and Mr Grayling being on rival camps in the debate over Britain's EU membership.
"So here we have the great axis of the Prime Minister and the Leader of the House," Mr Bercow is heard to say of the pair.
The PM shot back: "I don't know about that. Well, we're on the same side most of the time."
Asked whether Mr Cameron regretted his comment, a Downing Street spokesman said: "Both leaders have been invited to the summit because they are driving the fight against corruption in their countries. The UK stands shoulder to shoulder with them as they do so."
The spokesman declined to say whether the Nigerian or Afghan governments had contacted Downing Street following the Prime Minister's remarks.
He made clear that the PM was aware that he was being filmed at the time he spoke, telling reporters: "The cameras were very close to him. There were multiple cameras in the room."