Pc Toby Rowland issued a letter of claim against Mr Mitchell on Tuesday, in the wake of their differing accounts of a heated confrontation in Downing Street in September last year.
It is understood the officer was already considering legal action before Mr Mitchell said last week he wanted Mr Rowland to give his account of the argument under oath.
A Police Federation spokesman said: "We can confirm that Pc Toby Rowland yesterday issued a letter of claim for libel against Andrew Mitchell MP. This relates to comments he made following the media-termed Plebgate incident in Downing Street in 2012.
"It would be inappropriate to comment further."