Top WA sprintcar driver Ryan Farrell has been stripped of his speedway licence and faces a suspension after failing a random drug test.
Farrell, 38, admitted the positive result and apologised to fans in a statement after the World Series Sprintcars round in Bunbury on Friday.
The seven-time State champion will front a Speedway Australia tribunal in the next month where he could face a two year ban and a $2000 fine.
Speedway Australia competitions and operations manager Paul Trengove declined to confirm the substance, saying it was prohibited but not necessarily "performance enhancing".
Mr Trengove said Speedway Australia had a zero tolerance policy for prohibited substances and was determined to maintain the sport's integrity and safety.
Farrell said he had been dealing with depression and drug use for a long time and was now receiving counselling.
Farrell said he was adamant he had not taken performance enhancing or recreational drugs while behind the wheel.
Farrell has been in hot form this season, winning nine of his 11 races before Friday's test.
In 2010, he was convicted of two minor drug possession charges in the Northern Territory after a routine traffic stop by police.