Steve Hilton urged politicians not to get too complacent and highlighted a survey by The Economist to suggest the UK is more corrupt than Brazil, France, the USA and other countries.
Mr Cameron made the embarrassing gaffe on Tuesday during a conversation with the Queen ahead of his anti-corruption summit.
Mr Cameron could be heard highlighting Nigeria and Afghanistan as “possibly the two most corrupt countries in the world”, in footage on ITV News showing him chatting in a group including the Archbishop of Canterbury and Commons Speaker John Bercow.
The Prime Minister 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.”
Mr Hilton was Mr Cameron’s advisor until he left his post and launched the crowdsourcing website Crowdpac. Reacting to Mr Cameron’s on camera comments, he wrote:
He was apparently referring to The Economist’s “crony capitalism index”, which ranks countries by the billionaires within it who amassed their wealth “thanks to their chumminess with government”.
Afghanistan’s President Ashraf Ghani has remained silent on the Prime Minister’s comments but Nigeria’s President Muhammadu Buhari said he was “shocked and embarrassed” by the remarks.
Mr Buhari was due to give the keynote speech at the summit in an address entitled: “Why We Must Tackle Corruption Together”.
Downing Street said the Prime Minister was aware of “multiple cameras in the room” when he spoke.
A spokesman said that Mr Buhari and Mr Ghani “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.”
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