(AFP) - World heavyweight champion Tyson Fury announced he was retiring from boxing in a profanity-laced tweet on Monday.
"Boxing is the saddest thing I ever took part in, all a pile of s**t, I'm the greatest and I'm also retired..." Fury wrote on Twitter.
Reports by US broadcaster ESPN last week claimed Fury had tested positive for cocaine. The British fighter recently cancelled an October bout with Wladimir Klitschko to defend his WBA and WBO heavyweight belts.
Contacted by AFP, Fury's promoters, Hennessy Sports, did not wish to comment.
Fury, 28, had been scheduled to earn the biggest purse of his career in a rematch with Klitschko at the Manchester Arena after shocking the Ukrainian last November to become world champion.
However, Fury withdrew from that date as his camp claimed he had been "declared mentally unfit to fight".
ESPN reported Fury tested positive after providing a urine sample to the Las Vegas-based Voluntary Anti-Doping Association (Vada) in Lancaster, England on Sept 22.
Cocaine, while an illegal drug, is not banned in boxing if taken out of competition.
Fury later tweeted a link to a YouTube video in which he said George Kokkalenios, an American lawyer, was "telling the truth about the corruption in boxing".
The video showed Kokkalenios at the wheel of a car, talking to an unidentified person behind the camera and suggesting Klitschko was somehow involved in the cocaine reports about Fury.
"What's going on with Tyson is really a major travesty," he said.
"He's getting railroaded. He's been completely railroaded by crooked people. He's not on drugs - I know that for a fact. He's not taking anything, he's not doping. The system is corrupt and that's it.
"Klitschko, he is definitely involved in this because he's got everyone in his pocket. I'm not saying he's a bad man at all, but I can tell you this for a fact: his wealth carries a lot of influence."
Fury appeared to make light of the reports in a previous Twitter post on Saturday.
He tweeted a picture of Tony Montana, played by Al Pacino in the film Scarface, sitting at a table with a pile of cocaine in front of him.
Fury imposed his own face onto the picture, which he captioned "#Tysonmontana".
Fury, the self-styled 'Gypsy King', also won the IBF belt in his fight against Klitschko, but had to forfeit it after failing to fulfil a mandatory bout against Vyacheslav Glazkov.
A previously scheduled rematch with Klitschko, set for July 9, was scrapped after Fury sustained an ankle injury.
Following the cancellation of that fight, it emerged that UK Anti-Doping (UKAD) had charged Fury and his cousin, Hughie, with a doping offence. They deny wrongdoing.
The sample, taken nine months prior to Fury's sensational victory over Klitschko, contained traces of the banned substance nandrolone.
Fury was provisionally suspended, but his ban has since been lifted and his legal team have threatened action against UKAD.
He is due to face a hearing in November and could have been stripped of his titles if found guilty.
British promoter Eddie Hearn recently said Fury would never fight again, prompting his trainer and uncle, Peter Fury, to claim his nephew would be back in action next year.
If Fury sticks to his decision to quit the sport, he will bow out unbeaten, having won all 25 of his professional contests with 18 stoppages.
His greatest moment, defeating Klitschko in Germany after the Ukrainian was unbeaten for over a decade, also proved his downfall.
Fury has courted almost constant controversy since winning the world title, angering many with a series of highly sexist, anti-semitic and homophobic comments.
With Fury retired, his titles are expected to become vacant, increasing the likelihood Klitschko will fight for them.