Hardaker, 22, pulled out of the World Cup earlier this week, and will now be expected to report for pre-season training with the Rhinos on November 25.
Chief executive Gary Hetherington said in a statement: “Brian McDermott and myself have now had the opportunity to discuss the matter with the England management and with Zak himself and we now have a better understanding of the events leading up to Zak’s departure from the England squad.
“We believe Zak acted unprofessionally and in doing so, breached the Leeds Rhinos Player Code of Conduct.
“We will continue to monitor Zak’s future conduct and help him deal with his own issues; which are more to do with a lack of professionalism than anything else.
“Zak has apologised for the embarrassment caused to his family and Leeds Rhinos and he is desperately sorry for the situation he finds himself in.”
Hardaker hadn’t featured in England‘s first two World Cup games against Australia and Ireland, although he did put in a man of the match performance for England Knights against Samoa the week before the tournament.
He said: “I have paid a heavy price for my own actions and will now miss out on my dream of helping England win the World Cup. I am sorry for any embarrassment I have caused and my focus now is on moving forward next season and putting these events behind me.”