Charr fails test, cancels WBA defence

World heavyweight champion Manuel Charr says he doesn't understand why or how he could have failed a recent drug test.

German boxer Manuel Charr (L) has failed a recent drug test and cancelled his WBA title defence. (AAP)

Heavyweight champion Manuel Charr will not defend his WBA belt next week against American challenger Fres Oquendo after a 'suspect' doping test by the Syrian forced the fight to be cancelled, his management says.

Charr's A sample from an August 31 test showed "two suspect substances" and due to time constraints regarding the opening and testing of the B sample the bout was now cancelled.

"I cannot explain it and I am asking myself how could it happen," the fighter, who lives in Cologne, told the Koelner Express newspaper.

"I have never taken anything. I fought Vitali Klitschko and Alexander Povetkin, seven title fights and I was always a clean athlete.

"Why should I take something before the fight against Fres Oquendo."

The 33-year-old Charr (31-4) had won the WBA title against Russia's Alexander Ustinov last year.