Justin Gatlin has said he was "shocked and surprised" when he learned that his coach Dennis Mitchell allegedly offered to supply performance-enhancing drugs.

In a statement on his Instagram account, the reigning 100 metres world champion also denied doping and threatened legal action against anybody who claims otherwise.

Gatlin's comments come after undercover reporters from the Daily Telegraph filmed Mitchell and another member of the American sprinter's entourage, athletics agent Robert Wagner, allegedly offering to obtain banned drugs for an actor training for a film.

Mitchell and Wagner have both denied wrong-doing.

"I am not using and have not used PEDs (performance-enhancing drugs)," wrote Gatlin, who has twice served doping bans during a controversial career.

"I was shocked and surprised to learn that my coach would have anything to do with even the appearance of these current accusations. I fired him as soon as I found out about this.

"All legal options are on the table and I will not allow others to lie about me like this.

"I have no further comments as it is now a legal matter. They will next hear from my lawyer."