Brock Lesnar, the Ultimate Fighting Championship’s massive heavyweight titleholder, was forced to withdraw Monday from his highly anticipated Nov. 21 title defense in Las Vegas against Shane Carwin.

Carwin, via his Twitter feed, said the fight will be rescheduled for UFC 108 on Jan. 2.

UFC president Dana White confirmed the news to Yahoo! Sports. White said Lesnar has been ill for more than three-and-a-half weeks and unable to train. With the bout at UFC 106 less than a month away, Lesnar was forced to postpone the title defense.

“He said he’s never been this sick in his life,” White said. “He said it’s been going on for a long time and he just hasn’t been able to shake it.”

Lesnar’s co-manager, Brian Stegeman, said he does not know whether Lesnar had H1N1, commonly known as swine flu, and that he is beyond the stage at which he could be tested for the virus. He said Lesnar had upper respiratory issues, fever and extreme fatigue.

The fight matches two of the biggest and most powerful heavyweights in the UFC. Lesnar (4-1), a former World Wrestling Entertainment star, was a junior college and NCAA Division I wrestling champion. Carwin is a former Division II national wrestling champion.

Both men weigh around 295 pounds and cut weight to make the heavyweight division’s 265-pound limit.

Carwin (11-0), who made several appearances in Los Angeles on Saturday to promote the fight, was clearly disappointed at getting the news.

“My heart just sank, as you can imagine,” Carwin told Yahoo! Sports upon hearing the news about the fight postponement.

The UFC announced Monday afternoon it will move the previously scheduled Tito Ortiz vs. Forrest Griffin fight, a rematch of an epic 2006 contest won by Ortiz on a controversial split decision, into the Nov. 21 main event slot

Stegeman said Lesnar just took an entire week off from training with no appreciable improvement. He said Lesnar’s head mixed martial arts coach, Marty Morgan, didn’t think he’d have enough training time to properly prepare.

“We kept thinking that he’d get better and be back at it, but it finally got to a point here where we were looking at a situation where Brock would have just two weeks to train for this fight.

“That’s just not fair to him. There’s no way a two-week training camp is even close to being fair to him and we all realized that. We kept hoping Brock would respond and get better, but it just lingered.”