Cabinet Secretary Sir Jeremy Heywood has insisted that he did a "competent" job during his investigation into the 'Plebgate' incident involving former Chief Whip Andrew Mitchell and police officers at Downing Street.
He said he told Mr Cameron that CCTV footage showed "inaccuracies and inconsistencies" in emails sent by someone who claimed to have seen the altercation last September.
Sir Jeremy said this meant they should not be relied upon in deciding whether Mr Mitchell should be sacked.
When challenged by members of the Public Administration Select Committee, about his suitability to carry out an inquiry into the incident, he said:
"It's a perfectly legitimate part of my role and frankly, I think I did the job competently and came to the right conclusion."
Britain's most senior police officer has promised a "ruthless" investigation into an alleged conspiracy against Andrew Mitchell.
Andrew Mitchell has written a piece in which he has accused the police of trying to destroy his career in a smear campaign.
Police investigating the row involving Andrew Mitchell have arrested a second man today. See how the "plebgate" events unfolded.