A senior civil servant who investigated the "plebgate" affair tells MPs he advised that emails supporting claims Andrew Mitchell swore at police should not be relied upon in determining his fate.

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Cabinet secretary Sir Jeremy Heywood told the House of Commons public administration committee that he did a "competent" job and came to the right conclusion when asked to investigate emails and CCTV footage relating to the incident at the Downing Street security gates.

However MPs seemed underwhelmed by the evidence from the country's top civil servant. When he said that he had not looked into whether the word "pleb" had been used, the mood was incredulous.

Asked whether it occurred to anyone in Downing Street that there could have been "massive fabrication" to undermine the Chief Whip, Sir Jeremy replied:

"We accepted there were unanswered questions, including the possibility of a gigantic conspiracy or a small conspiracy. Those were unanswered questions. But we decided, on balance, to let matters rest as they were, decided to stick by Andrew Mitchell, keep him in post and move on."

Sir Jeremy 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, which meant they should not be relied upon in deciding whether Mr Mitchell should be sacked as government chief whip.

Sir Jeremy told the committee that the prime minister had only asked him to look into the emails and it was not his role to investigate the police.

He confirmed that he did not meet the sender of the emails, did not ask to see the police log and did not pass on his concerns about the emails to the police.

When asked why he did not pass on these concerns to the police, Sir Jeremy said: "I flagged up to him (David Cameron) the unanswered questions and we had a discussion and the conclusion was to let the matter lie."

Sir Jeremy has come under fire for failing to question the account of the incident entered in a police log by officers guarding Downing Street at the time, which claimed Mr Mitchell called them "morons" and "plebs" - something the Sutton Coldfield MP denied.

Resignation

While Mr Cameron insisted that the chief whip should not lose his job over the incident, as he had apologised to police for swearing, Mr Mitchell eventually resigned in October after weeks of controversy.

A Channel 4 Dispatches programme later revealed that the person who sent the emails claiming to be a passing member of the public was in fact a serving police officer, while CCTV footage of the exchange between Mr Mitchell and the police officer appeared to contradict claims in the police log that several members of the public witnessed the incident. A Metropolitan Police inquiry is currently under way into the emails.

You are not equipped to carry out the right investigation. You didn't get to the truth about that email Bernard Jenkin

Sir Jeremy faced challenges from committee members, including chairman Tory MP Bernard Jenkin, over whether someone in his position should even have been asked to carry out an inquiry into the incident.

Mr Jenkin told him: "You are just the wrong figure to conduct such an investigation. Is the cabinet secretary the correct person to conduct such an investigation in the midst of intense media furore, while there are contested accounts on which the career of a senior minister turns?

"You are not equipped to carry out the right investigation. You didn't get to the truth about that email.

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False allegations

"You lost a minister because of false allegations about him that were not properly investigated."

But Sir Jeremy insisted that Mr Mitchell's departure was not as a result of his inquiry, and said it was not for him to suggest to the prime minister that his probe should be widened to look into the veracity of the police account.

Frankly, I think I did the job competently and came to the right conclusion Sir Jeremy Heywood

The cabinet secretary told the committee: "It's a perfectly legitimate part of my role and frankly, I think I did the job competently and came to the right conclusion."

Asked why he did not consult the police log and challenge its version of events before reporting back to the PM, he said: "I can only do what I am asked to do. It's not the role of a civil servant or the cabinet secretary to start investigating the police.

"That's not my job. I don't have the powers, I don't have the expertise, it wouldn't be right for the cabinet secretary to be involved in that sort of thing.

The cabinet secretary said that once police investigations are complete, Downing Street would "take stock" of its response to the Mitchell allegations.

"If it is proven in a court of law - if it gets to that - that someone has tried to falsify evidence to bring down a cabinet minister, that is a very fundamental issue, so I think it would be wrong to rush to judgment now."