Oscar De La Hoya is not a fan of the proposed showdown between boxing icon Floyd Mayweather and UFC superstar Conor McGregor. And that's putting it mildly.

De La Hoya, one of boxing's most significant promoters and a 10-time world titleholder in six weight classes, issued an open letter to fans on Thursday slamming the proposed fight between Mayweather, who plans to exit retirement for the bout, and McGregor, the biggest star in mixed martial arts.

"To my fellow boxing fans, I write in the hopes that together we can protect the sport of boxing," De La Hoya wrote. "With each passing day, it looks more and more likely that the circus known as Floyd Mayweather vs. Conor McGregor will be coming to town in the near future. As undercard fights start to take form, athletic commissions give their blessings in exchange for millions of dollars, and the fighters start counting even more cash, one group will eventually be left to make sure this farce doesn't occur -- we, the fans, who are the lifeblood of our sport."

De La Hoya lost a split decision and his junior middleweight world title to Mayweather in a 2007 fight that set several box office records for boxing that are now owned by the May 2015 fight between Mayweather and Manny Pacquiao. De La Hoya wrote that he believed boxing was only beginning to "dig out of the hole" left by the Mayweather-Pacquiao fight "that ended up being as dull as it was anti-climactic."

He said he fears that a Mayweather-McGregor fight would inflict more damage on a sport that is enjoying a banner year in 2017 thanks to a series of popular fights, including Anthony Joshua-Wladimir Klitschko, Keith Thurman-Danny Garcia, Gennady Golovkin-Daniel Jacobs and Canelo Alvarez-Julio Cesar Chavez Jr. Golden Boy is also promoting an Alvarez-Golovkin showdown on Sept. 16.

"But if you thought Mayweather/Pacquiao was a black eye for our sport -- a matchup between two of the best pound-for-pound fighters that simply didn't deliver -- just wait until the best boxer of a generation dismantles someone who has never boxed competitively at any level -- amateur or professional," De La Hoya wrote. "Our sport might not ever recover.

"I fully understand the initial attraction from any fan of combat sports. McGregor is almost certainly the best pound-for-pound MMA fighter. Floyd is Floyd -- the most dominant boxer of his time. But success in one sport does not guarantee success in another. Far from it. And let's be clear, these are two different sports -- from the size of the gloves fighters wear, to the size and shape of the ring, to the fact the one sport allows combatants to use their legs to strike.

"Think about it, beyond Bo Jackson and Deion Sanders, what other athlete has successfully competed in two sports in the modern age? And Jackson and Sanders both played both baseball and football throughout their high school and college careers before going professional. Furthermore, it's not like McGregor would be fighting a good fighter, let alone a mediocre one. He would be fighting the best. To use a bit of an extreme analogy, I happen to be a pretty good golfer. Could I potentially hold my own on one of the second-tier tours? Maybe. But would I be able to compete with Rory McIlroy, Jordan Spieth or Sergio Garcia? Of course not. Nor would I think to try."

The 40-year-old Mayweather (49-0, 26 KOs) has not fought since easily outpointing Andre Berto in September 2015 and then retiring, but he has said he would exit retirement for only one fight -- the big one against McGregor, who very much wants the fight and would face Mayweather in his first boxing match.

"To my fellow boxing fans, I write in the hopes that together we can protect the sport of boxing. With each passing day, it looks more and more likely that the circus known as Floyd Mayweather vs. Conor McGregor will be coming to town in the near future. As undercard fights start to take form, athletic commissions give their blessings in exchange for millions of dollars and the fighters start counting even more cash, one group will eventually be left to make sure this farce doesn't occur -- we, the fans, who are the lifeblood of our sport." Oscar De La Hoya, in an open letter on a proposed Floyd Mayweather-Conor McGregor fight

McGregor (21-3), 28, recently applied for a boxing license in Nevada, where the match with Mayweather likely would be held later this year, though there is no set date yet. UFC president Dana White and McGregor have come to terms on their end of the deal, and White said he has plans to meet with Al Haymon, Mayweather's adviser, to try to reach terms with Mayweather.

"Now, I know critics will say that I'm only writing this letter because my company is promoting what will be the culmination of an outstanding boxing year when Canelo Alvarez takes on Gennady 'GGG' Golovkin in September, and I don't want anything to distract attention away from that fight," De La Hoya wrote. "But my interest is in the health of boxing as a whole. It always has been. And if Floyd were to come out of retirement to take on someone like Keith 'One Time' Thurman, Errol Spence or some other top welterweight, not only would I applaud the fight, I'd be the first one on line for a ticket. That kind of fight is what the fans -- and I am a fan first -- deserve. Which brings me back to the circus."

De La Hoya ripped Mayweather-McGregor as nothing more than a money grab.

"Floyd's and Conor's motivation is clear. It's money," De La Hoya wrote. "In fact, they don't even pretend it's not. But it's also a lack of consequences for when the fight ends up being the disaster that is predicted. After this fight, neither of them will need us anymore. Floyd will go back to retirement -- presumably for good this time with another nine-figure paycheck -- and Conor will go back to the UFC. It's a win-win for them. It's a lose-lose for us. We'll be $100 lighter and we will have squandered another opportunity to bring boxing back to its rightful place as the sport of kings. At this point, only we can shut the circus down by making it clear that we won't pay to see a joke of a fight and telling our casual-fan friends that they shouldn't either."