Former Chief Whip Andrew Mitchell has called for a full inquiry into the "plebgate" row after it was alleged a police officer falsely claimed to have witnessed the event.

The officer is alleged to have posed as a member of the public, and claimed to have witnessed the incident at Downing Street in an email to his local MP, John Randall - Mitchell's under-study as deputy chief whip.

The 'witness' claimed he was passing through Downing Street with his nephew, and heard Mitchell called the police "plebs" - the pejorative reference at the centre of the current row. He added that passers-by outside the gates were shocked, and some may have filmed the confrontation

Randall passed on the complaint to senior Conservative figures. Despite denying using the word "plebs", Mitchell resigned after weeks of pressure from the media.

The complaint received by Randall was backed up by the official police log, which claims "several members of public [were] present" at the time of the incident. However newly emerged CCTV footage, acquired by Channel 4 News, shows there was only one member of the public at the scene. Furthermore, this witness did not appear to take too much interest in the exchange.

Mitchell, who always denied calling the officers "plebs" after they refused to let him cycle through the Downing Street gates on 19 September, resigned nearly a month after the incident.

He has long maintained he was the victim of a "stitch up" and has now demanded a full investigation.

In his first full interview since the incident, Mitchell told Channel 4 News: "I always knew that the emails were false, although extremely convincing. It has shaken my lifelong support and confidence in the police.

"I believe now there should be a full inquiry so we can get to the bottom of this."

A spokesman for No 10 said of the latest claims: "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."

Although John Tully, chairman of the Metropolitan Police Federation, said his body "unequivocally and categorically refutes any allegation that it was part of a conspiracy to unseat a cabinet minister", the Met has now released a statement saying: "The allegation that a serving police officer fabricated evidence is extremely serious. It goes to the very heart of the public's trust in the police service."

Still from the CCTV footage of Andrew Mitchell's encounter with police (Dispatches/Channel 4 News)
Still from the CCTV footage of Andrew Mitchell\'s encounter with police (Dispatches/Channel 4 News)

London Mayor Boris Johnson said: "An allegation that a serving police officer posed as a member of the public whilst fabricating evidence is a matter of the utmost gravity.

"I know that the Metropolitan Police Commissioner is committed to establishing the truth here, as soon as possible."

A spokeswoman for The Police Federation of England and Wales said: "We are aware that there is an ongoing investigation into this matter, we are therefore unable to comment further at this stage."

The latest dispute comes just days after a Diplomatic Protection Squad officer on suspicion of misconduct in connection with alleged leaking of information to the media to the incident.

The officer, from the specialist Metropolitan police unit which guards embassies and members of the government, was arrested on 15 December and has been suspended from his duty.