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Thirty police are working on the investigation into claims that an officer gave false evidence over the "plebgate" affair, Scotland Yard says.

Cabinet minister Andrew Mitchell resigned after allegedly insulting police working in Downing Street.

But CCTV evidence and claims another officer sent an email falsely claiming to be a witness to the exchange have led to an inquiry being set up.

The Metropolitan Police say they regard the matter as "extremely serious".

that an officer sent an email, purporting to be from a member of the public who witnessed the row between Mr Mitchell and police in Downing Street in September, in which he was said to have called them "plebs".

The email was sent to Deputy Chief Whip John Randall, who passed it on to No 10. It and used "gutter language", adding that people watching were "appalled", with some possibly "inadvertently" filming what was going on.

But CCTV coverage suggests no-one other than the officers involved was within earshot of Mr Mitchell, the Conservative MP for Sutton Coldfield.

Mr Randall has given a statement to the police but has declined to comment further in public, saying he does "not wish to jeopardise the police's position".

´óÏó´«Ã½ political editor Nick Robinson said he had been told the wordings of the email and the statements made by the officers involved in the row were "remarkably similar".

A Diplomatic Protection Squad officer was arrested on Saturday for alleged misconduct in public office.

Mr Mitchell has always denied using the word "plebs" but has admitted he lost his temper and swore at the officers after they refused to let him cycle through the main gate.

He resigned from his post in October, following weeks of criticism in the media, but since the latest allegations he has called for a full inquiry.

In a statement, the Metropolitan Police said they were looking at whether there was "any evidence of a conspiracy between" the arrested officer and anyone else, and the circumstances surrounding the incident.

They are also investigating the leaking of information to the press.

The Met said: "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."

They added that they had "acted swiftly" and were conducting "a thorough and well-resourced investigation to get to the truth of the matter as quickly as possible".

The claim by the off-duty officer to have witnessed the incident has been referred to the Independent Police Complaints Commission.

At Prime Minister's Questions, David Cameron said: "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."

John Tully, chairman of the Metropolitan Police Federation, which represents officers in the force, "unequivocally and categorically" denied that his organisation had been "part of a conspiracy to unseat" Mr Mitchell.

However, Conservative MP David Davis, a friend of Mr Mitchell, told ´óÏó´«Ã½ Radio 4's Today programme there had been a "clear presumption" in the media that the police were "saints" and that the former chief whip was among "the sinners".

He added that the email should have been investigated in more detail by Cabinet Secretary Sir Jeremy Heywood, as this was "not exactly Sherlock Holmes" and that No 10 had made "daft decisions".

Mr Davis also said: "We've had a really serious injustice. He [Mr Mitchell] should return to high office."

But a Downing Street spokesman said: "The prime minister's clear view is that reasonable inquires were made."

He said that Sir Jeremy's investigation had included a review of the CCTV evidence, but not of the police log.

Mr Mitchell told Channel 4 News: "I always knew that the emails were false, although extremely convincing."

He added that his "lifelong support and confidence in the police" had been "shaken".