Sources: Amanda Nunes withdraws from UFC 256 for undisclosed reasons

Nunes dominates Spencer to retain featherweight title (0:29)

Amanda Nunes puts on a dominating performance against Felicia Spencer, landing heavy blows to the head throughout. (0:29)

Two-weight UFC champion Amanda Nunes has withdrawn from her featherweight title defense against Megan Anderson next month for undisclosed reasons, according to multiple sources.

Nunes (20-4) was scheduled to defend her 145-pound title against Anderson (10-4) in the main event of UFC 256 on Dec. 12 in Las Vegas. The UFC has not officially announced the cancellation of the bout or a new main event for what will mark its final pay-per-view of 2020.

Anderson, who is riding a two-fight winning streak and has not yet fought for a UFC championship, tweeted a series of broken-heart emojis from her account on Monday.

"All I was told was she has a 'serious medical issue' and that they're working on rescheduling the bout currently," Anderson added later in a statement. "Obviously, it sucks because we've been working so hard for this, but I hope she has a speedy recovery and the goal remains the same. Nothing has ever come easy for me. This is just another part of the journey and we'll be ready for whatever date they set. Our time is coming one way or another."

Widely considered the greatest female fighter of all time, Nunes holds the UFC 135- and 145-pound titles. She has not lost since September 2014. Nunes and her partner, UFC strawweight Nina Ansaroff, celebrated the birth of a daughter in September.