Floyd Mayweather Jr will not fight UFC star Conor McGregor "in a million years" and the bout would be an "an embarrassment for boxing", according to 10-time world champion Oscar de la Hoya.
Speculation of a fight between the pair has been rife since McGregor was granted a boxing licence in California in early December.
Since then the pair have traded verbal barbs, with talks on a lucrative deal said to have begun for what would be a first-of-its-kind event - boxing legend versus mixed martial arts superstar.
Mayweather has insisted he is "happily retired" but recently told the Irishman "let's make it happen", while McGregor has refused to let the issue lie, claiming he would "knock Mayweather's head clean off".
But De la Hoya is not a fan of the proposed bout, claiming it would be a complete mismatch.
"Boxing is boxing and MMA is MMA," De la Hoya, a former world champion in six weight divisions, told Omnisport.
"They're two separate sports. It would be a joke, it would be an embarrassment for boxing if an MMA fighter were to go into the ring, it would be a no contest.
"It would be the same thing if a fighter went into the octagon. It would be an embarrassment for MMA.
"I just feel that these are two separate sports and you have McGregor who is the best at what he does, and you have Mayweather who is the best at what he does. I don't see this fight taking place, not in a million years."
De la Hoya added that if McGregor is intent on facing Mayweather he should first earn experience in the amateur ranks before challenging one of the all-time greats, who has a perfect 49-0 record.
He added: "Not only are there safety issues, but if McGregor gets in a boxing ring he'll get outclassed easily. McGregor has never fought a boxer ever in his life. It's a whole different sport.
"When you cannot kick, when you cannot clinch, when you cannot do the armbar, when you cannot choke somebody out and you can only punch, it's a whole different sport, it's a whole different discipline.
"What McGregor should do, in all seriousness, is he should start off by fighting an amateur boxer, win a few amateur fights, maybe go on to win the Golden Gloves, and then when he turns pro he should fight a four-rounder, maybe he'll get beat, but he should get experience in boxing.
"Then call out the best. You cannot be calling out a top fighter in boxing right off the bat. It took me 20 years to win 10 world titles in six different weight classes. McGregor, just because he's the best in MMA, he cannot go out there and fight a boxer, it's unheard of."