Rugby Football League chief executive Nigel Wood has expressed concerns about the "implications for the game" if Denny Solomona's purported move to rugby union side Sale Sharks from Castleford Tigers goes ahead.
The 23-year-old Samoa international remains contracted to the Super League club until 2018, but he failed to report for pre-season training in November.
On Wednesday, Solomona was added to Sale's Champions Cup squad ahead of Saturday's round-three match against Saracens at Allianz Park, despite his signing not having been confirmed.
"This has profound implications for the game," Wood told reporters. "Clearly the sanctity of contracts has to be respected.
"It's a concern for more than the game, it's bigger than just rugby league or rugby union, it's almost like a law of the land issue really.
"Unilaterally walking away from things is not something you would condone. We've been in contact with both the RFU and Premier Rugby to express our concerns. They are aware of our position."
Solomona broke Lesley Vainikolo's record for the most tries in a Super League season as he crossed 42 times in 2016.