However, Gatlin's legal team told the paper that the agent had only represented the runner a few times and suggested it was likely the two were "putting on quite a show to impress who they thought were "Hollywood" producers".
The Telegraph report published Monday said the undercover reporters traveled to Gatlin's Florida training camp and presented that they were making a film about athletics, and that they needed help training a male actor so he could get in shape.
It had claimed Mitchell, the American former medal-winning sprinter who was previously banned for doping, and sports agent Robert Wagner had offered to supply performance-enhancing drugs to undercover reporters.
In a statement to the Telegraph, Mitchell, who confessed on oath to using banned substances during his career, also exonerated his athletes. "It was just big talk".
"I did not actually source or supply the substances the reporters asked for but stupidly claimed I could".
World 100 metres champion, Justin Gatlin, said on Tuesday he was "shocked and surprised" at doping allegations made against his coach, Dennis Mitchell.
"I fired him as soon as I found out about this", Gatlin wrote.
Gatlin took to his Instagram account to proclaim his innocence in the matter, vowing that "I am not using and have not used PEDs". The agent ¬- who is said to have worked only briefly with Gatlin - also denied being involved in doping.
Gatlin has rejected the allegations, for which he is considering suing the newspaper, and has sacked Mitchell, who won 4x100m relay gold for the US in 1992 before receiving a two-year doping ban in 1998.
He added: "All legal options are on the table as I will not allow others to lie about me like this".
Sebastian Coe, president of the International Association of Athletics Federations (IAAF) which set up the AIU, has called the allegations "extremely serious".
"We are presently co-ordinating with the Athletics Integrity Unit in order to investigate these claims fully", USADA said in a statement. "They will next hear from my lawyer".