Was Tory Chief Whip framed? Police officer accused of posing as member of public to help force Andrew Mitchell's resignation

CCTV footage and claims of fabricated evidence may exonerate Mitchell

A police officer was accused yesterday of masquerading as a member of the public and lying to help force the resignation of the Government Chief Whip.

In an extraordinary development in the "Plegbate" scandal, it emerged that a man whose testimony was crucial to the resignation of Andrew Mitchell did not in fact witness his outburst.

The "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****** plebs", a Channel 4 News investigation suggests.

CCTV footage of the event released by Mr Mitchell last night also appears to contradict the police logs of the incident which stated there were "several members of the public present" at the time. It only shows one member of the public outside the gates – and no one who would match the description of the officer.

The claims emerged after a member of the diplomatic protection squad was arrested on suspicion of misconduct in a public office 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.

Attention will focus on the Police Federation – which represents rank and file officers – and whose campaign against Mr Mitchell was instrumental in leading to his resignation. It has strongly denied any "conspiracy".

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 account appeared to corroborate the police version of events, that Mr Mitchell had called the officers "f****** plebs" when they stopped him cycling through the main gates.

Last night Downing Street said: "Any allegations that a serving police officer posed as a member of the public and fabricated evidence against a cabinet minister are exceptionally serious. It is therefore essential that the police get to the bottom of this as a matter of urgency. We welcome the Metropolitan Police Commissioner's commitment to achieve that aim."

The Metropolitan Police announced at the weekend that a member of its Diplomatic Protection Group, the unit which guards Downing Street, had been arrested on suspicion of gross misconduct. The officer was suspended and bailed to appear next month.

The Dispatches investigation for Channel 4 News found that the "witness" had contacted Mr Randall by email – a day before the story was first 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.

Last night Mr Mitchell told Channel 4 News that the letter was "very convincing" but he was certain it was untrue. "It was clearly 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," he said.

Mr Mitchell vociferously denied the claims and Mr Cameron agreed to suspend his judgement while an investigation was carried out by the Cabinet Secretary. It examined the CCTV footage – now made public – which appeared to cast doubt on the witness's claim. As a result Mr Mitchell was allowed to stay in post until he decided to resign of his own accord around a month later.

However the Dispatches investigation established that the supposedly independent witness was in fact a serving police officer.

When the man was contacted by Dispatches he denied he was even there, saying: "I wasn't a witness to anything."

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

The Independent Police Complaints Commission said yesterday that it had received a referral from the Met and had decided to supervise the investigation. The Met Commissioner, Bernard Hogan-Howe, said the arrest did not discredit the Downing Street officer's log of the incident.

The Conservative MP Dominic Raab described the developments as "disturbing". He said: "Up until now, the assumption has been that the police had behaved like angels, and Andrew Mitchell a villain. It now appears that at least one officer has deliberately told falsehoods, explicitly designed to drag Andrew Mitchell's name through the mud. We need a swift and rigorous investigation to see whether he acted alone, was put up to it, or indeed acted in collusion with other officers."