Castleford Tigers have confirmed they intend to sue Denny Solomona for breach of contract after he left them to join Sale Sharks.
Sale confirmed they had signed Solomona - who scored a record 42 tries in Super League last season - on Tuesday morning.
Solomona has two years left on his contract and Castleford, supported by the RFL, had warned they would take legal action should the New Zealand-born player choose to switch codes.
And the Super League have launched legal proceedings against Solomona, Sale and the player's agent.
Tigers chief executive officer Steve Gill said: "If the shoe was on the other foot and Solomona had suffered a career-ending injury in the last match of the season there would have been an absolute obligation on us to honour our contractual obligations.
"We look after our players and fully expect our players to respect their contractual commitments. Sadly and with regret Solomona has done the polar opposite and effectively and unilaterally walked away from the remaining two years on his contract.
"In August this year we made it very clear to Solomona and to Sale Sharks that there was an unbreakable contract in place until the end of season 2018 with no basis whatsoever for either ourselves or Solomona to depart from each other's mutual obligations.
"The idea of losing the player was not welcomed by us and we have repeatedly told Sale Sharks that it would be impermissible for them to talk to Solomona let alone sign him. Clear unequivocal warnings have clearly been disregarded.
"Unfortunately we are not privy to what has happened over the last three months between Solomona and his agent but on the back of losing the player in the circumstances described and now announced by Sale Sharks we confirm that today we have issued Proceedings out of the High Court in Leeds for damages against Denny Solomona for breach of contract (dated November 20, 2015) and for damages including exemplary damages against Sale Sharks Ltd & Andrew Clarke [Solomona's agent] for inducing breach of the said contract."
Castleford say that should they lose their legal battle, the implication for rugby league and all other sports where contracts are used are immense.
Gill added: "We are fighting this not just for Castleford Tigers but also for the integrity of all sports including of course rugby union clubs. Our belief is that all contracts are sacrosanct in professional and amateur sports."