Hardaker missed the Super League Grand Final on Saturday after breaching club rules.
He was dropped just before the game with little detail offered by the club and Castleford were then beaten by Leeds Rhinos at Old Trafford.
And now it has emerged he was banned for a failed drugs test and he has been left out of the England side for the Rugby League World Cup.
An RFL spokesperson said: “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 following a Super 8s game between Castleford Tigers and Leeds Rhinos on September 8, 2017.
“He will therefore not be considered for the England Rugby League World Cup squad due to be announced today at 12pm.
“The Rugby Football League will be making no further comment until the outcome of the case has been determined.”
England assistant coach Denis Betts confirmed he had been left out of the squad.
“He’s been the form full-back in the competition,” Betts told the BBC. “He was pivotal to how Castleford have played this year.
“He’s a fantastic rugby league player and he would have been fantastic for us.”
It is understood Hardaker tested positive for cocaine and could be hit with a two-year ban.
Cocaine use out of season incurs smaller bans, with St Helens full back Ben Barba serving a 12-game ban after testing positive late last year.
But in-competition positive tests usually warrant two-year bans, although that can be reduced or extended to four years depending on circumstances.
Castleford and Hardaker are expected to comment later.