Heavyweight champion Lennox Lewis of Britain was ordered to give up his WBA belt if he fights Michael Grant on April 29.
US District Judge Lewis Kaplan made the ruling today after having presided over a civil trial last month that included arguments by lawyers for Lewis and promoter Don King.
The judge said going through with the Grant fight would cause Lewis to breach a contract he had signed for a fight against Evander Holyfield.
The judge said his ruling preserves "Lewis' freedom to fight whomever he wishes provided he first surrenders the title, but subjects him to the risk of punishment for contempt of court if he does not comply with his obligations."
Lewis is scheduled to fight Grant in Madison Square Garden. The case came about when Lewis filed a civil lawsuit against King, seeking a declaratory judgment on his behalf.
Kaplan said the case raised the question "whether a renowned professional athlete is bound in his commercial dealings by the same standards of honesty and fairness aspired to by the rest of society."
A lawyer for Lewis did not immediately return a telephone call for comment.