After taking on a shark (sort of), swimming legend Michael Phelps has a new challenge in his sights – racing Conor McGregor

McGregor switched from mixed martial arts to boxing in order to take on one of the pound-for-pound greats in Floyd Mayweather Jr last month and Phelps, the most decorated Olympian of all time, would like to see the Irishman swap his gloves for goggles.

The 23-time Olympic gold medallist tweeted a mocked up graphic of himself facing off with the UFC superstar in the aftermath of his fight with Mayweather.

While Phelps insisted it was initially just a joke, he stated he would be happy to race McGregor in the pool and even offered him a 50-metre head start.

"Everyone kept tweeting and retweeting this photo that I tweeted of him and I across from each other," Phelps said.

"I said it as a joke. If he wants to swim, I don't care. I'll race him.

"If we did it in a year, and I trained for a year, or even six months, I could probably go 48 seconds if I had to. I don't see him breaking a minute, so I could probably beat him with a 50-metre head start.

"I would definitely exercise the option to have a conversation if he wants to swim."

Phelps raced a computer-generated shark in July and lost, but he did beat NBA legend Shaquille O'Neal in 2009.