WBA "regular" heavyweight champion Manuel Charr has failed a doping test and cancelled his clash with Fres Oquendo.
Germany-based Charr (31-4-KO17) had been due to defend his belt against Puerto Rican Oquendo (37-8-KO24) on September 29 in Cologne but that fight has now been called off.
Book Joshua v Povetkin now
Early booking lines are open for the Wembley Stadium heavyweight showdown
The 33-year-old failed a test under Voluntary Anti-Doping Association (VADA) conditions on August 24 but he retains the right to appeal and have his "B" sample tested.
Charr had been touted as a possible opponent for Anthony Joshua, who is the current WBA "super" champion and defends his belt against Alexander Povetkin on Saturday, live on Sky Sports Box Office.
In a statement given to Sky in Germany, Charr said: "I swear I have not taken anything.
"I have had seven title fights in my career, have undergone countless rehearsals and NEVER had a positive test. Of course, I have taken supplements in preparation, but never done anything forbidden.
"I eat in different restaurants every day, maybe something happened that I did not know anything about, unfortunately, I cannot give an explanation, I'm just shocked and sad."
Povetkin is one of four fighters to have beaten Charr, knocking the German out in 2014, and he has since been defeated by Johann Duhaupas and Mairis Breidis.
He took the WBA "regular" title against Alexander Ustinov last November and the clash with Oquendo was scheduled to be his first defence.
Watch Joshua vs Povetkin, at Wembley Stadium, on September 22, live on Sky Sports Box Office. Book via your Sky remote or book it online here.
Even if you aren't a Sky TV subscriber you can book and watch it at skysports.com/boxofficelive.