Police constable Toby Rowland has launched legal proceedings against Andrew Mitchell, a former chief whip and one-time international development minister from Prime Minister David Cameron's Conservative party.
Rowland claimed last year that Mitchell used abusive words against him and other officers guarding Cameron's office as Mitchell tried to push his bicycle through the gates of Downing Street.
Mitchell admitted swearing, but denied using the politically toxic comments attributed to him. The resulting scandal, however, forced him to resign a few weeks later.
Last week, after prosecutors said they would not charge Rowland, Mitchell directly accused the policeman of lying and said he would to try to make him give evidence on oath in the case.
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A spokeswoman for the Police Federation, the police union, said: "We can confirm that PC Toby Rowland yesterday issued a letter of claim for libel against Andrew Mitchell.
"This relates to comments he made following the media-termed plebgate incident in Downing Street in 2012."
Prosecutors said last week they had insufficient evidence to show that Rowland lied in his account of the altercation with Mitchell, while the police officer's superiors in the Metropolitan Police have also decided not to discipline him.