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A police officer's claim that he witnessed a row outside Downing Street involving ex-chief whip Andrew Mitchell is being probed by the Independent Police Complaints Commission (IPCC).

It said it was "considering the validity of the officer's claim", which he apparently made to his local MP.

The police constable was arrested on suspicion of misconduct in a public office on Saturday.

Mr Mitchell is alleged to have called officers "plebs" during the argument.

In a statement, the IPCC said: "The Independent Police Complaints Commission is supervising an investigation by the Metropolitan Police Service into an allegation of misconduct in public office.

"The circumstances surround a serving police officer's claim to have independently witnessed a widely reported incident outside Downing Street in central London on 19 September 2012.

"The investigation is considering the validity of the officer's claim, which is understood to have been made to a Member of Parliament in a private capacity.

"The investigation is linked to enquiries by the MPS [Metropolitan Police Service] into how internal MPS information was obtained by national newspapers following the incident.

"The IPCC received a referral from the MPS today [Monday] and following an assessment of the available information, a decision was made to supervise the matter."

'Serious questions'

The Met has previously said that the arrested officer was "not on duty at the time of the incident in Downing Street" and has found no evidence to suggest those on duty on the day were involved in the unauthorised release of information.

The watchdog can choose to supervise some parts of an investigation while leaving others, such as where it considers something to be an internal disciplinary matter, in the hands of the force.

It was claimed that on 19 September, Mr Mitchell made the comments and swore at police when they asked him to use the pedestrian gate, rather than the main gate, to leave Downing Street.

The former chief whip later apologised for not having shown enough respect to the police, but has always maintained that he "did not use the words attributed to me".

Mr Mitchell resigned from the government over the affair a month later.

The official police log of the row was allegedly leaked and published by the Daily Telegraph and the Sun newspapers.

The Diplomatic Protection Squad officer has been suspended from duty and bailed to return in January.

The Metropolitan Police Commissioner, Bernard Hogan-Howe, has backed the police officers who were on duty during the Andrew Mitchell incident.

Mr Hogan-Howe said he had seen "nothing that causes me to doubt that original account".

Speaking on LBC, the Commissioner said he hoped the inquiry into the other officer - who was not on duty at the time - would be resolved in "a matter of days" - before Christmas.

The Commissioner said the investigation into the officer had not affected the original account of officers at the scene.