Conor McGregor could lose millions before he even steps into the ring for Sunday's fight against Floyd Mayweather.

Ahead of Saturday's weigh-in, Mayweather has warned McGregor that he must reach the weight limit of 154 pounds or pay a significant penalty.

Mayweather was concerned McGregor appeared to be heading into the fight over the limit, saying the Irishman was "extremely heavy" and had only a couple of days to lose the weight.

The fight would proceed as scheduled if McGregor missed weight but Mayweather would seek to recoup a chunk of his opponent's purse.

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Mayweather told Fight Hype: "Conor McGregor is extremely heavy right now, extremely heavy.

"I think he's 164, so he's still got 10 pounds to go. You go get that extra money if you can't make the weight, get them extra millions ready [UFC president] Dana [White], somebody got to pay a fine.

McGregor responded by posting a photo from his training camp, saying "the final rev of the engine" of his conditioning programme was underway.

The final rev of the engine on the @conormcgregorfast conditioning program tonight! What a camp it has been!

A post shared by Conor McGregor Official (@thenotoriousmma) on

But Mayweather was still skeptical.

"He got to get that weight down like a true champion, a true champion is discipline, and very responsible," Mayweather told Fight Hype. "I don't think he gonna make the weight, even if he do make the weight, that's even better.

"But if he don't make the weight, we still gonna fight but it's going to be a heavy fine - get me that money."

Who said it: McGregor v Mayweather edition