Andrew Mitchell is set to sue the Sun for libel over its report that claimed he had called Downing Street police officers "plebs" during an argument as he left Number 10.

The Tory MP's lawyer has sent the Sun a "letter before action", the first stage in litigation, putting them on formal notice that he intends to sue over its scoop last September which revealed the row Mitchell had with police over taking his bicycle through the main gates.

The decision to sue the paper is part of Mitchell's fightback which may also involve actions against other newspapers, his lawyer Graham Atkins said.

"We are seeking damages, an apology, an undertaking that the words complained of are not repeated and costs," Atkins told the Guardian.

In a Dispatches documentary on Channel 4 screened on Monday night, Mitchell told of his frustration with Downing Street's handling of the "plebgate" furore, claiming he had been "stitched up" and that Cameron just wanted him to "lie low" rather than clear his name.

Mitchell, who was forced to resign in October after an intense backlash following claims he called police officers protecting the prime minister "plebs" after they refused to let him ride his bicycle out of the main gate at Downing Street.

"I think Downing Street wanted this to go away. They really wanted me to lie low and let them get on with running the country but I couldn't do that - I couldn't wake up every morning for the rest of my life knowing that I had been stitched up," said Mitchell.

He admits that he swore in the presence of police but has always denied he used the word "'pleb". The Sun and other newspapers reported that an official police report showed he branded the officers "plebs".

It wasn't until December that the "pleb" part of the story was challenged when CCTV footage threw into doubt the police log and an email purporting to be an eyewitness account of the event emerged, which may clear his name of some of the claims. An investigation was immediately launched by Scotland Yard and three police officers have been arrested in connection with the scandal.

Mitchell believes he would still be in government if footage of the exchanges had been released swiftly.

Of suggestions initial briefing notes were altered to include the word pleb, Mitchell told Channel 4: "I understand that two of the three poisonous phrases are in it, but whether there were two or three or one, whether it was done within one minute or an hour or three hours of the incident at the gate, it is wholly and totally untrue."