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The police watchdog - the Independent Police Complaints Commission (IPCC) - is investigating "the validity" of a claim made by a police officer whose evidence helped lead to the resignation of Andrew Mitchell as the government chief whip.

Mr Mitchell resigned after he apologised for swearing at an officer who refused to open the main gates of Downing Street to let the chief whip through on his bicycle.

He has always insisted that claims that he called police officers "f...ing plebs" were untrue, although he has admitted to saying: "I thought you guys were supposed to f...ing help us."

The IPCC tonight said that an officer who was arrested on Saturday on suspicion of misconduct in public office had claimed to have "independently witnessed" the incident even though the Metropolitan Police have stated that he was "not on duty at the time of the incident in Downing Street".

The statement raises serious questions about whether there was ever any independent corroboration of the account of the claims that Mr Mitchell called police officers plebs.

I understand that Channel 4's Dispatches programme have been working for some time on an investigation into alleged discrepancies into the accounts of the incident which led to the resignation of a senior minister.

Mr Mitchell believes that his version of events will eventually be proved to be true.