The Archbishop of Canterbury intervened to defend the President of Nigeria after the British Prime Minister told the Queen his country was "fantastically corrupt".

Justin Welby corrected David Cameron at a reception at Buckingham Palace to mark the 90th birthday of Queen Elizabeth II.

Cameron was talking to the Queen about an international anti-corruption summit he is hosting in London tomorrow. He said: "We had a very successful cabinet meeting this morning, talking about our anti-corruption summit.

"We have got the Nigerians – actually we have got some leaders of some fantastically corrupt countries coming to Britain. Nigeria and Afghanistan - possibly two of the most corrupt countries in the world." The Queen did not respond.

However, Archbishop Welby, referring to Nigeria's president Muhammed Buhari, said: "But this particular president is actually not corrupt."

The encounter was followed by another controversial conversation in which the Queen was caught on camera saying Chinese officials were "very rude" during last year's state visit by President Xi Jinping.

However, on the issue of corruption in Nigeria, both the Queen and Cameron will realise that Archbishop Welby talks from a depth of huge expertise in this area.

Nigeria is one of Archbishop Welby's areas of particular interest. Before ordination he worked in the oil industry and after he left the business world and entered the Church, he was made a Canon of Coventry Cathedral in 2002, leading on its ground-breaking reconciliation work. This took him to Africa and the Middle East and he became a sought-after expert in Nigeria. He still works with groups involved in ongoing conflict in the north. Also in the Niger Delta, he has worked on reconciliation with armed groups. He has lectured on reconciliation at the US State Department. 

Buhari and Afghan President Ashraf Ghani have pledged to clean up corruption in both their countries. Buhari said he was "shocked" and a spokesman for the Nigerian president said Cameron's remarks did not reflect the good work he was doing.

Afghanistan is at number 166 in the Corruption Perceptions Index which ranks countries, beaten only by North Korea and Somalia. Nigeria is at 136.