Greg Norman has criticised the PGA Tour and commissioner Jay Monahan about their stance on players wanting to join the proposed Saudi-backed breakaway golf league.
Rory McIlroy said the potential league was "dead in the water" on Sunday, following Dustin Johnson and Bryson DeChambeau joining the likes of Tiger Woods, Jon Rahm and Collin Morikawa in pledging their future to the PGA Tour.
Monahan reiterated in a player meeting this week that those wanting to join the Saudi Golf League would face bans by the PGA Tour, while Norman - fronting the SGL - accused the commissioner of "bullying" players with "unenforceable" threats.
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Brooks Koepka said ahead of the Honda Classic that some players would still "sell out" and join the Saudi circuit, with Norman insisting that "this is just the beginning" at the end of his 500-word letter.
The letter, obtained by Sky Sports News, said: "Dear commissioner Monahan: Surely you jest. And surely, your lawyers at the PGA Tour must be holding their breath.
"As has been widely reported, you have threatened the players on the PGA Tour, all of whom are independent contractors, with lifetime bans if they decide to play golf in a league sponsored by anyone other than the Tour.
"For decades, I have fought for the rights of players to enjoy a career in which they are rewarded fully and properly for their efforts. They are one-in-a-million athletes. Yet for decades, the Tour has put its own financial ambitions ahead of the players, and every player on the tour knows it.
"The Tour is the Players Tour not your administration's Tour. Why do you call the crown jewel in all tournaments outside the Majors "The Players Championship" and not "The Administration's Championship?"
"But when you try to bluff and intimidate players by bullying and threatening them, you are guilty of going too far, being unfair, and you likely are in violation of the law.
"Simply put, you can't ban players from playing golf. Players have the right and the freedom to play where we like. I know for a fact that many PGA players were and still are interested in playing for a new league, in addition to playing for the Tour. What is wrong with that?
"What is wrong with allowing players to make their own decisions about where to play and how often to play? What is so wrong with player choice? Why do you feel so threatened that you would resort to such a desperate, unwise and unenforceable threat?"
Norman quoted an article by Alden Abbott, the former general counsel of the Federal Trade Commission, in which the author claims that a ban would trigger a "slam-dunk antitrust lawsuit".
"Competition in all aspects of life, sport, and business is healthy and the players deserve to be well compensated, which is why so many players have expressed an interest in playing in a new league," Norman added.
"But when you threaten to end players' careers and when you engage in unfair labour practices with your web of player restrictions, you demonstrated exactly why players are open minded about joining a league that treats players well, respects them, and compensates them according to their true worth.
"Commissioner - this is just the beginning. It certainly is not the end."
Sky Sports News has contacted the PGA Tour for comment and is awaiting a response.