Conor McGregor has demanded $100million in cash to face Floyd Mayweather Jr in a fanciful cross-combat mega-fight.

McGregor, fresh off becoming the first fighter to hold two UFC championships simultaneously, and Mayweather have teased a  bout ever since the 49-0 boxing legend hung up his gloves following a victory over Andre Berto last November.

The most recent outburst from 'The Notorious' comes on the heels of Mayweather telling a photographer that comparing the two men is "disrespectful".

And that has prompted MMA's biggest talker to respond, McGregor accusing Mayweather of running scared.

"Floyd's not ready for this," McGregor told a crowd at a New York nightclub in a video obtained by TMZ. 

"Much respect to Floyd. He's a solid businessman on what he's been able to do. 

"He's a f****** animal at what he's been able to do. But as far as real fighting - as far as true, pure, unarmed combat - Floyd don't want none of this.

"He wants it under boxing rules. He wants a boxing match. He doesn't want a fight," McGregor said. 

"I want $100million cash to fight him under boxing rules because he's afraid of a real fight."