Bonnar was a black belt in both Brazilian jiu-jitsu and taekwondo, and was recognised as an emblematic figure in MMA.

The American died at Desert Springs Hospital in Las Vegas.

Bonnar's 2005 bout with compatriot Forrest Griffin is regarded among fans as one of the greatest fights in MMA history.

It is said to have been pivotal to the success of the Ultimate Fighting Championship (UFC), with the body's President Dana White describing it as the "most important fight in UFC history".

"That fight with Forrest Griffin changed the sport forever," White said, as reported by L'Équipe.

"Stephan Bonnar was one of the greatest fighters in the Octagon.

"The fans loved him, identified with him.

"He will be missed and never forgotten."

Bonnar eventually lost the fight but remained friends with Griffin and named his son after him.

"Stephan was a lot of things - he was always the most interesting person in the room, he had the biggest heart and most importantly, he was my friend," Griffin wrote on Twitter.

"I always loved it that people got excited when they found out we were really friends.

"I'll always miss you, brother."

Bonnar retired from MMA competition in 2012 before being inducted into the UFC Hall of Fame alongside Griffin the following year.

After putting his career as an athlete to one side, he was appointed as President of North Star Combat, a Minnesota-based MMA promotion company.