All 800 members of the Metropolitan Police's diplomatic protection group are to be questioned as officers investigate the Andrew Mitchell "plebgate" row.

A team of around 30 officers will obtain statements from every member of the elite unit, known as S06, which protects government ministers and includes the officers allegedly called "f*****g plebs" by Mitchell outside Downing Street.

The inquiry, codenamed Operation Alice, is expected to cost around £64,000 and up to 2,500 man hours.

The investigators will attempt to find out whether there was a conspiracy against former chief whip Mitchell, who resigned in the wake of the plebgate controversy.

Doubts emerged about the police's official version of events when a SO6 officer was arrested over allegations he had falsely claimed to be a member of the public who witnessed the event. CCTV footage obtained by Channel 4 News also cast doubt on several other claims recorded in the police report and witness statements.

The inquiry follows on from a pledge by Met chief Sir Bernard Hogan-Howe to hold a "ruthless search for the truth" following allegations of a smear campaign and cover-up.

But John Tully, the chairman of the Metropolitan Police Federation, has branded the decision to quiz every single officer as a "waste of time and resources".

He said: "It is not unusual for any officers who were on duty at the time of an incident to be required to provide what's called a duty statement.

"But the Met have asked everyone in that command to make one, which is unusual.

"There are upwards of 800 people in that command and they have all had to put pen to paper in some way.

"We have a lot of officers who live quite a long distance from where they work, and if they were sitting at home in Norwich when Andrew Mitchell was allegedly saying these words it seems to be a waste of time and resources."

The extent of the investigation has also been criticised by Labour MP for Birmingham Steve McCabe, who told the Daily Mirror: "To spend this time and money questioning officers who can't possibly know anything is madness.

"It seems an awful lot of resources are being used when it would be much simpler if Andrew Mitchell just came clean and told us exactly what did happen."

Mitchell admits to swearing at the police officers after they refused to allow him to ride his bicycle through the main gates, but denies using the word plebs.

CCTV footage appears to contradict the police's version of events, notably a comment which suggested "several members of public [were] present" at the time of the incident and were "visibly shocked". However, the footage shows there was only one member of the public at the scene, who did not appear to take too much interest in the exchange.

The CCTV footage also suggests the row went on for a shorter period of time than the witness statements claim.

Scotland Yard said: "The allegations are extremely serious and it is therefore vital that a thorough and proportionate investigation is carried out.

"It continues to be progressed with urgency, determination and an open mind. We are not prepared to discuss who we may or may not want to speak to in relation to any investigation."