Video report by ITV News' Political Correspondent Libby Wiener
David Cameron has been overheard telling the Queen that Nigeria and Afghanistan are "possibly two of the most corrupt countries in the world".
The PM was speaking to the monarch about the countries coming to an upcoming anti-corruption summit in the UK, during an event at Buckingham Palace to mark the Queen's 90th birthday.
He told an audience, which included the Archbishop of Canterbury Justin Welby and Commons Speaker John Bercow:
"We had a very successful cabinet meeting this morning. We talked about our anti-corruption summit. We've got...leaders of some fantastically corrupt countries coming to Britain.
"Nigeria and Afghanistan are possibly two of the most corrupt countries in the world".
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.
A Number 10 spokesman said: "In the essays that both President Buhari and President Ghani contributed to the PM’s Against Corruption book for the summit, both leaders acknowledge the depth of corruption in their countries.
"In his essay, Pres Ghani acknowledges that Afghanistan is ‘one of the most corrupt countries on earth’.
"While Pres Buhari writes that corruption became a ‘way of life’ in his country under ‘supposedly accountable democratic governments’."