Cameron praises Nigeria and Afghanistan's corruption action
Nigeria and Afghanistan have taken "remarkable steps forward" on corruption, David Cameron told MPs - a day after calling the countries "fantastically corrupt".
The PM said the countries' leaders were "battling hard" to tackle the problem.
It comes after he was recorded talking to the Queen at Buckingham Palace.
An Afghan official said "bold" action had already been taken while Nigeria's president said he would not be demanding an apology from the PM.
Asked ahead of the anti-corruption conference in London if Nigeria was "fantastically corrupt", Nigerian President Muhammadu Buhari, who came to power last year on a promise to fight corruption, replied: "Yes."
Mr Buhari said he was more interested in the return of stolen assets held in British banks, adding that corruption in Nigeria was endemic and his government was committed to fighting it.
"What would I do with an apology? I need something tangible," Mr Buhari said, referring to efforts to recover the money.
The Afghan embassy in London said tackling corruption was one of President Ghani's top priorities and "bold" action had been taken.
"We have made important progress in fighting systematic capture in major national procurement contracts and are making progress on addressing institutional issues as well as issues related to impunity... therefore calling Afghanistan in that way is unfair."
Mr Cameron's original comment about the two countries came while he was speaking at an event to mark the Queen's 90th birthday about hosting world and business leaders at this week's anti-corruption summit in London
"We've got some leaders of some fantastically corrupt countries coming to Britain... Nigeria and Afghanistan, possibly the two most corrupt countries in the world," he was caught on camera saying.
Mr Cameron was asked about his comments during Prime Minister's Questions, where he jokingly checked his microphone was working, referred to "tips on diplomacy" and said he had made "many unforced errors" in the past 24 hours.
Answering a question from Tory backbencher Philip Davies - who asked why UK aid was being given to countries the PM sees as corrupt - he praised the action taken by Afghanistan and Nigeria and warned cutting off aid could "come back to haunt us here".
He also defended action by his own government, including on overseas tax havens and measures to make sure "plundered money from African countries can't be hidden in London".
In the footage showing Mr Cameron's comments, Archbishop of Canterbury Justin Welby intervened to say: "But this particular president is not corrupt... he's trying very hard," before Speaker John Bercow said: "They are coming at their own expense, one assumes?"
Earlier, Foreign Secretary Philip Hammond said the PM had been "merely stating a fact" in his comments, and ex-London mayor Boris Johnson said people would "find it refreshing he was speaking his mind".
In Transparency International's 2015 corruption perception index, Afghanistan was ranked at 167, ahead of only Somalia and North Korea, Nigeria was at 136.
Transparency International said the success of Thursday's summit would be judged on whether concrete action was agreed on tackling tax evasion and secrecy in the wake of the Panama Papers disclosures, stopping tainted firms from bidding for public contracts and protecting whistleblowers who expose corruption.
US Secretary of State John Kerry - who is representing the US at the summit - said the fight against corruption should be made a global priority "of the first order".
"Today the cost of corruption globally is exploding, and it's exploding everywhere," he told the Oxford Union. "Despite recent progress, as a global community, we just are not doing nearly enough to eliminate this scourge - and that needs to change."
Labour has suggested a Tory government "hosting an anti-corruption summit was like putting the fox in charge of the chicken coop".
"The government is refusing to take meaningful action to close Britain's constellation of tax havens, which together constitute the largest financial secrecy network in the world," said shadow international development secretary Diane Abbott.