Australian UFC champion Robert Whittaker has been ruled medically unable to compete in what was meant to be his title defence on February 11.

Australian middleweight champion and headline act Robert Whittaker has apologised to his fans after being forced to withdraw from Perth's UFC blockbuster in February.

Whittaker was scheduled to take on former champion Luke Rockhold in UFC 221 at Perth Arena on February 11.

But the 27-year-old has been ruled out of his title defence with an undisclosed injury.

Yoel Romero will take Whittaker's place in what will now become an interim title bout against Rockhold.

"I would like to apologize to all my supporters and fans of MMA for not being able to put on the show you deserve," Whittaker wrote on his Instagram account.

"I've been ruled medically unable to compete and won't be able to defend on home soil.

"This card meant so much to me and I'm devastated for not being able to fight.

"All I can do is focus on getting healthy to come back better than ever. All the best to Yoel and Luke for their fight and I'll be waiting."

The Perth event is already a sell-out.

Whittaker became the first undisputed UFC champion from Australia when Canadian Georges St-Pierre vacated the middleweight title.in December.