The allegation that a serving police officer fabricated evidence relating to the Andrew Mitchell "plebgate" row prompts calls for a full and independent investigation into the case.

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Following revelations from Channel 4 News on Tuesday that the row that engulfed Andrew Mitchell was allegedly fuelled by a police officer who posed as a member of the public, there was cross-party support for a full police investigation.

Scotland Yard said 30 officers were now working on the case, while David Cameron told the Commons: "A police officer posing as a member of the public and sending an email potentially to blacken the name of a cabinet minister is a very serious issue and does need to be seriously investigated."

Both Mr Mitchell's former cabinet colleague Vince Cable, of the Liberal Democrats, and Labour's shadow home secretary Yvette Cooper called for an independent investigation.

Ms Cooper said: "These are very serious allegations involving a police officer and must be rigorously investigated. And the full facts must be set out.

"None of the reports from the government in September raised the possibility of a witness nor is it clear whether this was checked out at the time or had any influence on events.

'Importance of trust'

"Andrew Mitchell admitted swearing at the police. The Met Commissioner is standing by the accounts from the police officers present at the gate, which Andrew Mitchell has now said are untrue.

"The prime minister was urged repeatedly to set up a proper investigation to get to the facts and refused to do so, and it is incredible that, three months on, we still don't have clarity about what happened.

This must not be allowed to drag on any longer Yvette Cooper

"Given the seriousness of this issue and the importance of trust between the police and government ministers, this must not be allowed to drag on any longer."

In a statement in response to the revelations, the Metropolitan Police said it wanted to "get to the truth of the matter as quickly as possible".

The statement said: "The Metropolitan Police Service is conducting a thorough and well-resourced investigation to get to the truth of the matter as quickly as possible.

"Since receiving fresh information last Thursday, the MPS has acted swiftly. On Saturday, an investigation into the alleged unauthorised disclosure of information was reopened. On Saturday, an officer was arrested on suspicion of misconduct in public office.

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False claims

"On Sunday, the investigation was referred to the IPCC (Independent Police Complaints Commission) for supervision."

The investigation seeks to identify the source of the leak, whether this originated from the Met or a third party, the circumstances surrounding the police officer's claim to have witnessed the incident and any evidence of a conspiracy.

The investigation is described as large-scale, currently involving 30 officers. A Met statement added: "The investigation will not be short"

Tuesday night's Channel 4 News broadcast allegations that a police officer posed as a member of the public and falsely claimed to have witnessed an exchange between Andrew Mitchell and a police officer at the gates of Downing Street.

The officer is said to have written to his local MP giving details of the then chief whip's behaviour when he was blocked from cycling out of the main gates in Downing Street.

The email said Mr Mitchell had sworn repeatedly and called the officers on guard "plebs". It also suggested that passers-by outside the gates had been shocked, and some may have filmed the confrontation.

Leaks

The account closely matched that in the official police log, which was later leaked to the Daily Telegraph.

Number 10 said the claims - which emerged after a member of the diplomatic protection squad was arrested - were "exceptionally serious".

But in the Commons later, Prime Minister David Cameron described the current police investigation as "thorough and well-resourced" and urged MPs to allow it to get to the truth.

The chair of the House of Commons home affairs committee, Keith Vaz, has called on Met Police Commissioner Bernard Hogan-Howe to offer an explanation of how information about the altercation got into the public domain and why the Downing Street police log "appears to conflict with CCTV evidence".

Mr Vaz, who said the latest revelations raise "serious issues of concern", also asked for an explanation of "the actions of the police officer who apparently claimed to provide information about the scene at which he was not present, and how he obtained this information".

In a letter to Mr Hogan-Howe, Mr Vaz wrote: "I would be grateful if you would let me know who will be conducting the inquiry and the timetable as to when it will report, also who will be entitled to see the conclusions.

The Metropolitan Police Federation, declined to comment on the ongoing investigation, but added: "It is important to note that we called for a full inquiry at the time of the original incident.

"This was declined by both New Scotland Yard and Downing Street.

"We welcome and agree with Commissioner Bernard Hogan-Howe's comments that, despite any questions that have arisen, there is no reason to doubt the accounts of officers directly involved in the incident."