Luis Ortiz has failed a drugs test, according to WBC president Mauricio Sulaiman.
The unbeaten Cuban was due to challenge WBC champion Deontay Wilder for his title in New York on November 4.
However, Sulaiman said on Twitter that the Voluntary Anti-Doping Association had confirmed the 38-year-old tested positive for a banned substance.
VADA is yet to release details of Ortiz's test, and ESPN has reported that Wilder vs. Ortiz has been cancelled as a result.
Promoter Lou DiBella, meanwhile, told BBC Sport: "As of now the fight isn't cancelled but it's difficult to see it going ahead.
"Deontay will be very disappointed with the news. It is one of the biggest fights of his career potentially down the drain."
Ortiz previously tested positive for a banned substance after beating Lateef Kayode in 2014, causing that result to be overturned to a 'no contest'.
