David Cameron met Andrew Mitchell on Monday evening, fuelling rumours the former chief whip may be set for a return to the top of politics.

According to the Press Association, the prime minister, who is on a trip to Afghanistan, praised Mitchell's "calm and rational" response to the plebgate affair.

And Cameron said Tory MPs had "a lot of sympathy" for their colleague's situation.

He also disclosed that he met Mitchell in Number 10 on Monday - the day before Channel 4 News aired allegations that a serving policeman posed as a member of the public and falsely claimed to have witnessed his now notorious spat in Downing Street.

Cameron's comments, made to journalists during a trip to Afghanistan, came as the police continue to investigate the possibility that the ex-Cabinet minister was the victim of a conspiracy.

Yesterday a 23-year-old man was arrested and bailed by officers investigating incident.

The man, who is not a police officer or a member of police staff, was arrested at his home on Wednesday evening for encouraging or assisting an offence and taken to a London police station, before being released on bail until January 2013 on Thursday morning. He is the second person to be arrested as part of the inquiry.

"Officers investigating the circumstances surrounding a police officer's claim to have witnessed an incident in Downing Street in September 2012 have arrested a man on suspicion of intentionally encouraging or assisting the commission of an indictable offence on or around 14 December 2012," police said.

On Wednesday Scotland Yard on Thursday vowed to "get to the truth" of allegations that a police officer falsely claimed to have witnessed the "plebgate" row that led to Mitchell being forced to quit as chief whip.

The officer is said to have written to his local MP, posing as a member of the public, giving details of the altercation that took place when the senior Tory attempted to cycle out of the main gates in Downing Street.

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

The Metropolitan Police Service said on Wednesday that “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. If any evidence emerges of conspiracy this will form part of the investigation.

"This is a fast-moving and comprehensive investigation and the highly unusual events of the last three days have shown this inquiry will need to go where the evidence takes us as it progresses."

It came after CCTV footage of the incident was broadcast for the first time and appeared to contradict parts of a leaked police log about the spat.