The Brit picked himself off the canvas to win by unanimous decision at the O2 Arena on Saturday night and claim the interim WBC heavyweight title.
Victory also made The Body Snatcher, 31, the mandatory challenger for Deontay Wilder’s world crown.
But that status is under threat after the shock revelations.
It is reported that the ‘A sample’ taken by UK Anti-Doping before the fight was flagged for “one or more” banned substances.
The ‘B sample’ will now be tested.
According to Boxing Scene, the positive test result was reported to Whyte’s camp and the British Boxing Board of Control three days before the fight.
It is claimed that neither the WBC nor Rivas were made aware of the failed test.
The BBBoC allowed the bout to go ahead and as yet have not sanctioned the Brixton boxer.
Whyte, who is part of the WBC’s ‘Clean Boxing Program’, has the right to appeal the result.
The astonishing allegation was revealed by renowned boxing writer Thomas Hauser, who has a number of high-level contacts in the sport.
Whyte weighed in heavier than usual at 259 pounds for the Rivas bout and was knocked down in the ninth round before winning a 115-112, 115-112, 116-111 decision.
Promoter Eddie Hearn responded to the claims by insisting Whyte was cleared to fight by the BBBoC, although he did not deny his boxer has failed a test.
Hearn tweeted: "Further to reports I can confirm that both Dillian Whyte and Oscar Rivas were subject to extensive VADA and UKAD testing for their bout.
"Both fighters were cleared to fight by both bodies and the BBBofC."
Whyte was previously banned for two years in 2012 when he tested positive for Methylhexaneamine following a knockout victory over Sandor Balogh.
He claimed he had not knowingly taken the drug, which was contained in a nutritional supplement.
He returned to the ring in 2014 and has been involved in blockbuster fights against Anthony Joshua - his only defeat - Joseph Parker and Dereck Chisora.
Speaking after beating Rivas, Whyte said: "This is my 27th professional fight and he’s undefeated and has beaten some tough guys.
“I wanted to have a war. He’s a great fighter and I have a lot of respect for him
“I went to move, crossed my leg and got caught. But I got straight back up and won every round after that.
“We’ll see what happens. A lot of things get said in boxing but not a lot happens.
“Mandatory could mean I have to wait another 600 days!
“We are looking to fight November or December time after a rest. Everyone has written me off but I keep showing up and outboxing top fighters.”
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