Mitchell calls for inquiry after officer 'lied to help force resignation'

Andrew Mitchell last night called for a full inquiry after a police officer was accused of masquerading as a member of the public and lying to help force the resignation of the former Government Chief Whip.

In an extraordinary development in the "Plebgate" scandal, it emerged that a man whose testimony was crucial to the resignation of Mr Mitchell did not in fact see his outburst. The "independent witness" was in fact a serving police officer who was nowhere near Downing Street at the time Mr Mitchell was accused of calling police officers "f***ing plebs", a Channel 4 News investigation suggests.

CCTV footage of the event released by Mr Mitchell appears to contradict police logs which stated there were "several members of the public present" at the time. It shows only one member of the public outside the gates – and no one who would match the description of the officer. The claims emerged after an officer was arrested by police investigating the leaking of information relating to the altercation.

Last night, Downing Street issued an angry statement describing the new revelations as "exceptionally serious". Government sources expressed deep concern over the suggestions that there might have been a co-ordinated effort by police to discredit Mr Mitchell. The Metropolitan Police Federation last night strongly denied any "conspiracy".

Attention will focus on the Police Federation – which represents rank-and-file officers – whose campaign against Mr Mitchell was instrumental in leading to his resignation.

The "witness" is understood to have contacted his local MP, John Randall – who was then Mr Mitchell's deputy in the Tory whips' office – and claimed to have been passing by the gates of Downing Street with his nephew when Mr Mitchell's altercation with officers took place. He told Mr Randall he had been outside Downing Street hoping to catch a "glimpse of a famous politician" and watched on in "horror" as Mr Mitchell "shouted obscenities" at the police officers.

The investigation for Channel 4 News found the "witness" had contacted Mr Randall – a day before the story was published in The Sun – claiming to be a passer-by. Mr Randall is said to have alerted Downing Street to the claims, which – when backed up by the police logs – appeared to confirm Mr Mitchell's guilt. The account appeared to corroborate the police version of events, that Mr Mitchell had called the officers "f***ing plebs" when they stopped him cycling through the main gates.

Mr Mitchell – who claims he never used the word "pleb" to describe the officer – admitted swearing but insists parts of a police log of the incident published in the media were "false".

Last night, Mr Mitchell said the email was "aimed to destabilise me and finish me off by sending it into the heart of government to my deputy, and could easily have done so very fast."