The police officer at the centre of the Plebgate row that cost Andrew Mitchell his cabinet job is to sue the former chief whip for libel.

Toby Rowland, the officer on duty in Downing Street at the time of the altercation, insists Mitchell called him a pleb. Mitchell accused him of lying as part of his own libel appeal against the Sun.

Now Rowland is to launch a libel action of his own against Mitchell.

The libel action was confirmed by the Police Federation. Rowland had been planning to sue Mitchell before the MP held a press conference last week, where the former chief whip and his lawyers repeatedly said Rowland was lying when he claimed he had been called a pleb.

Last week, the Crown Prosecution Service concluded there was insufficient evidence to suggest Rowland was lying.

That prompted Mitchell to say that he was prepared to state on oath that Rowland's version of events was not true, and he challenged Rowland to tell his story on oath.

At the press conference last week, Mitchell had said this would happen when his libel action against the Sun came to court. He called Rowland's statements "toxic", "lies", and "lying phrases". He repeated his contention that he had in fact said to the police: "I thought you guys were supposed to f-ing helping us."

MP David Davis, a close political ally of Mitchell, said: "We have been trying to get Pc Toby Rowland into the witness box to face the evidence and cross-examination under oath, and if this law suit by any means accelerates that, so be it."

Alison Saunders, the director of public prosecutions, said that was not sufficient evidence "to show that Mitchell was the victim of a conspiracy of misinformation".

Saunders said one officer was to be charged with misconduct in public office. The officer, PC Keith Wallis, 53, allegedly sent an email to the deputy chief whip purporting to be a member of the public who had witnessed the incident.

Bernard Hogan-Howe, the Metropolitan police commissioner, said no action would be taken against Rowland.

Rowland's handwritten notes of the encounter, plus records of a conversation he held with his superior immediately afterwards, were examined by the police and the CPS as part of the inquiry. He has never been arrested.