As a new investigation is launched into allegations that Andrew Mitchell called police officers "plebs", Channel 4 News reveals the controversial email accusing the former chief whip of swearing.

It is a small word - but a hugely loaded one. And the accusation of its use by the former chief whip has been very damaging to the Conservative party.

An investigation by Channel 4 News and Dispatches has uncovered evidence that questions the original eyewitness account of Andrew Mitchell's use of the word "pleb" and the official account in the police log of the incident.

And now for the first time, Channel 4 News can reveal the email sent by the alleged eyewitness which backed up the account in the police log, as published in the Telegraph on 25 September - one week after the incident.

CCTV footage uncovered by the Channel 4 News/Dispatches investigation contradicts the eyewitness claim [see email above] that there were "several other tourists" present, and that the eyewitness was with his nephew.

There is only one pedestrian captured near the Downing Street gates on CCTV, and although his face is not visible, he does not appear to be particularly disturbed by whatever he is watching.

The investigation also sheds light on the man behind the email: he was a serving police officer. And crucially, he told Political Correspondent Michael Crick, twice, that he was not actually there.

See video below for the full report.

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