American sprinter Justin Gatlin has denied using performance-enhancing drugs after an undercover report by The Telegraph in London into alleged activities by Gatlin’s representatives led to the opening of a doping investigation.

 “I was shocked and surprised to learn that my coach would have anything to do with even the appearance of these current accusations,” Gatlin wrote in a post on his Instagram account . “I fired him as soon as I found out about this.”

The IAAF’s Athletics Integrity Unit said it is investigating the allegations in conjunction with the United States Anti-Doping Agency. The report was published in Tuesday’s edition of the British newspaper The Daily Telegraph.

“These allegations are very serious,” said Brett Clothier, head of the AIU, “and strike at the heart of the integrity of athletics.”

IAAF President Sebastian Coe said the allegations are “extremely serious.”

The Telegraph reported that Gatlin’s coach Dennis Mitchell and a track agent, Robert Wagner, met undercover reporters at a training camp in Florida and offered to supply and administer testosterone and human growth hormone for an actor training for a film, for a fee of $250,000.

The newspaper said Mitchell and Wagner were secretly recorded saying the use of banned substances in track was widespread.

Wagner called the newspaper report “deeply flawed” and said it was based on things he said that were not true.

“I made up the comments to impress them, led on by a make-believe scenario,” Wagner said in a statement to The Associated Press. “It was just big talk. I did not actually source or supply the substances the reporters asked for but stupidly claimed I could.”

Wagner also said he notified the Athletics Integrity Unit of the situation four weeks ago.

Gatlin previously served a four-year suspension for a 2006 positive test. He also served a one-year suspension while at the University of Tennessee for a stimulant said to be in his prescribed attention deficit disorder medication.

“I am not using and have not used PEDs,” Gatlin wrote in his Instagram post. “All legal options are on the table as 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.”

In July, the 35 year old won the 100 meters at the IAAF World Championships in London in what was former world champion Usain Bolt’s last competitive race.