Speaking through his spokesman, President Muhammadu Buhari suggested Mr Cameron had been referring to Nigeria’s past notoriety for corruption during the leadership of his predecessors.
The Prime Minister was caught on camera saying Nigeria and Afghanistan were “probably the two most corrupt countries” on the eve of a major corruption conference in London.
During the summit, Mr Buhari is due to give the keynote address entitled: “Why We Must Tackle Corruption Together”.
Footage obtained by ITV Newsshows Mr Cameron chatting to a group including the Queen, Archbishop of Canterbury Justin Welby and Commons Speaker John Bercow at an event to mark the Queen’s 90th birthday.
He told the Queen: “We had a very successful Cabinet meeting this morning to talk about our anti-corruption summit. We’ve got the Nigerians… actually we’ve got the leaders of some fantastically corrupt countries coming to Britain.
“Nigeria and Afghanistan, possibly the two most corrupt countries in the world.”
The Most Rev Welby is heard trying to intervene, saying “this particular president” is not corrupt himself.
Mr Buhari proclaimed a “war against corruption” when he took power last year.
During the election campaign, he made the issue one of his key priorities saying: “If Nigeria doesn’t kill corruption, corruption will kill Nigeria”.
He has arrested many high-ranking members of the political elite on corruption charges – though many of them are from the opposition People’s Democratic Party (PDP), which was ousted from power after 15 years in charge.
Downing Street insisted Mr Buhari had previously acknowledged that corruption became “way of life” in Nigeria under “supposedly accountable democratic governments”.
Afghanistan ranks at 166 out of 168 in Transparency International’s 2015 corruption index but Nigeria is 30 points higher at 136.