The former Leeds Rhinos star was omitted from the Tigers squad just three days before the Grand Final defeat to his former club.
Castleford boss Daryl Powell was forced to rejig his squad for the final at Old Trafford, with the club confirming he had breached the club's rules.
The 25-year-old failed a test after a Super 8s game against Leeds on September 8 and the RFL has now revealed he will be unavailable for selection for the World Cup in Australia.
A statement read: "The Rugby Football League can confirm that Zak Hardaker is provisionally suspended from all competition after it received notification from UK Anti-Doping that he had tested positive for a banned substance.
"He will therefore not be considered for the England Rugby League World Cup squad due to be announced on Monday at 12:00 BST.
"The RFL will be making no further comment until the outcome of the case has been determined."