The 25-year-old, a key player for his club, has been provisionally suspended from all competition after failing the test following a Super 8s game between Castleford and Leeds Rhinos on Sept. 8.
The RFL did not name the banned substance but media reports said it was cocaine.
England were due to announce on Monday their squad for the World Cup starting in Australia at the end of this month, and Hardaker had been expected to feature in it.
The player, who faces a potential two-year ban, apologised for his "enormous error of judgement" to his team mates, club and fans in a statement on the Tigers' website (www.castlefordtigers.com).
"I would like to make it clear that in no way did I, nor would I, ever take a substance with the intention of enhancing my performance," added Hardaker, who did not play in Saturday's 24-6 Super League Grand Final defeat by Leeds.
(Reporting by Alan Baldwin, editing by Ed Osmond)
