Amir Khan has revealed he suspects Canelo Alvarez could have been using performance-enhancing drugs when they met in the ring in 2016.
Alvarez tested positive for Clenbuterol in February in two separate drug tests and the Mexican is now facing a ban from the Nevada Athletic Commission.
The backlash to Alvarezs potential cheating forced the fighter to pull out of his rematch with Gennady Golovkin in May.
Khan faced and lost to Alvarez in May 2016, and suffered a brutal knockout at the hands of the powerful middleweight.
But after news of Alvarezs tests broke, Khan has admitted he thinks his opponent could have been cheating when they fought almost two years ago.
He said: Canelo had put on so much weight and grown so big when he caught me with that huge punch that I wondered then if something may be up. No excuses, because he beat me.
But if he was on anything its not right. If so, he should never be allowed to fight again. This sport is dangerous enough without drugs.
The initial reaction of sanctioning bodies to Alvarezs positive tests was alarming, with the WBC and WBA insisting the 27-year-old was innocent.
It seemed as if the incident may have been swept under the rug before Golovkin hit out at his opponent and accused him of taking drugs when they first fought last September.
The NAC have yet to rule on the issue, but Alvarez faces a one-year suspension, which could be reduced to six months for good behaviour.
Alvarez has protested his innocence and insisted the pair of failed drug tests were due to eating contaminated meat.
The tainted meat defence has been used successfully by Mexican-based fighters and athletes for several years.