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."
Police Commissioner Bernard Hogan-Howe has resumed his Christmas break, Scotland Yard said. Earlier today, the police chief broke off from his holiday to be briefed on the progress of the 'plebgate' investigation, which is being supervised by the Independent Police Complaints Commission.
The Cabinet Secretary Sir Jeremy Heywood has said that after Andrew Mitchell resigned he came back in November and asked to see the CCTV footage of his row with a Downing Street police officer.
When Mr Mitchell viewed the footage, he then asked for a copy to take away.
Conservative MP, Charlie Elphicke, then asked Sir Jeremy why he did not share the CCTV footage with Mr Mitchell before he resigned given that he had previously concluded there were discrepancies between it and the police log.
Sir Jeremy said:
I'm pretty sure that he was told the broad conclusions of my report and he didn't at that point ask to see the CCTV footage. If he had asked to see it, we would have let him see it.
The country's most senior civil servant will be cross-examined by MPs investigating the Andrew Mitchell "Plebgate" affair. Sir Jeremy Heywood will be asked about his role in finding out what really happened at the Downing Street gates.
The Sutton Coldfield MP was forced to resign as Chief Whip but has always denied calling police officers "plebs".
Met Police Commissioner Sir Bernard Hogan-Howe is due to give evidence to the Home Affairs Select Committee on the Andrew Mitchell "Plebgate" affair.
Keith Vaz, MP, Leicester East (Lab) says the commissioner has questions to answer.
What appears to have happened to Andrew Mitchell could well have been a Christmas special script. The chief whip had to resign following a 60-second 'incident' in, of all places, Downing Street.
Take a police officer apparently masquerading as a member of the public, a confidential log book finding its way into the public domain, add the results of the Hillsborough Inquiry, which have resulted in thousands of serving and former police officers being investigated, and the fact that 26 out of the 43 police forces do not have a permanent chief constable, and you have a dangerous cocktail.
In an article in the Sunday Times (£), former Chief Whip Andrew Mitchell has accused the police of destroying his career.
Mr Mitchell described the police log of the 'plebgate' affair as "toxic" and said he was a victim of "police elements".
He also accused the Police Federation of "not telling the truth" in its account of the incident.
The policeman at the centre of the 'Plebgate' row involving former Government Chief Whip Andrew Mitchell has been named by the Mail on Sunday newspaper.
The newspaper claims the officer can be seen on CCTV footage ushering Mitchell out of the Downing Street gates during the alleged 'Plebgate' incident and that he was also the author of the police log that documented the exchanges between the pair.
The Mail on Sunday also names the man who sent a letter of complaint about the incident to an MP.
The newspaper states he is related to a second man - a 23-year-old who is not a member of police staff - who has also been arrested over the affair.
Met Commissioner breaks off Xmas holiday citing concerns about welfare of 'plebgate' officers as a paper apparently identifies one.