A 23-year-old man, who is neither a policeman nor a member of police staff, is questioned as 30 officers investigate the case.

The 23-year-old was held on suspicion of intentionally encouraging or assisting the commission of an indictable offence on or around December 14.

He is not a police officer nor a member of police staff but the arrest relates to claims by an officer to have witnessed the clash in Downing Street, Scotland Yard said.

He was arrested on Wednesday night and questioned at a London police station before being released on bail to return in January.

The move comes after a serving police officer was arrested on suspicion of misconduct in public office in connection with the affair.

December 14 was the day after police received fresh information about the argument, on September 19, and one day before the officer was arrested.

The Met Police now have 30 officers working on an investigation into the row after widening their probe when new revelations emerged.

Mr Mitchell was forced to quit as Tory chief whip over the clash but insists he was the victim of a “stitch-up” and is pushing for a full inquiry.

Allegations that an officer posed as a member of the public and falsely claimed to have witnessed the argument are now being looked at by police.

They are also examining the possibility of a “conspiracy” to smear Mr Mitchell as part of a “large scale and complex investigation”.

And CCTV footage which appeared to conflict with the official police log of the altercation, which was leaked to the press, is being scrutinised.

Prime Minister David Cameron has described allegations that an officer tried to “blacken the name of a Cabinet minister” as a “very serious issue”.

The officer allegedly wrote an email to his local MP, Conservative John Randall, posing as a member of the public and accusing Mr Mitchell of calling police “plebs”.

The account in the email, which was written the day before the story first appeared in the press, was very similar to the police log which was also later leaked.

Littered with grammatical errors, spelling mistakes, capital letters and malapropisms, it was passed to Downing Street on September 25.

According to Channel 4 News, the writer claimed he had been walking past with his nephew and recognised Mr Mitchell.

“Imagine our horror when we heard MR MITCHELL shout very loudly at the police officers guarding YOU [expletive blacked out] PLEBES!! and “YOU THINK YOU RUN THE [expletive blacked out] COUNTRY” and just continued to shout obscenities at the poor police officers,” he wrote.

Mr Mitchell has admitted swearing at police but insists he never said the word “plebs” – a word that was seized on by Labour and the Police Federation.

The email allegedly helped fuel the row and keep up the momentum that eventually forced the MP to stand down several weeks later.

At the time, Cabinet Secretary Sir Jeremy Heywood compared the email’s claims to the CCTV footage. He decided they were not “reliable” but did not look at the police log.

There is speculation Mr Mitchell could be restored to the frontbench, depending on the results of the police investigation.

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