The first sample, which found mesterolone and methenolone acetate metabolites, was collected three days before the fight, which was held on July 15 in Michigan. Baumgardner had defended her undisputed title with a win over Linardatou.

Baumgardner has denied taking any illegal substances, adding that the sample provided after the fight on July 15 was negative, along with her sample on June 16.

"I never have, never would, and never will take this or any other drug. Not only would doing so be unethical, but it would also be completely contrary to how I've trained my entire career," Baumgardner said in a post on Instagram on Wednesday.

"As a professional athlete, I and I alone, am responsible for what I put in my body, and my body is a temple. I am also a proud role model for any young woman or girl who seeks to follow in my footsteps," she said.

"I have no intention of sitting back in silence and letting this 'play out.' I plan to keep you all updated every step of the way as I work to ensure my reputation as a clean athlete remains fully intact."

This is the second incident this month after British boxer Dillian Whyte returned "adverse analytical findings" in a random test, which led to the cancellation of his heavyweight fight with former world champion and compatriot Anthony Joshua.

Matchroom Boxing said a "full investigation" is being conducted in the Baumgardner case.

"Matchroom is committed to promoting a clean and fair sport for all athletes, and will assist Alycia and her team during this time, affording her the appropriate due process," Matchroom Boxing said on X, the social media platform formerly known as Twitter.