Britney Spears is being accused of lip-syncing recently during her Las Vegas show, according to the gossip blog On No They Didn't.
The blog says that the singer was lip-syncing during the song "Perfume." Reportedly, Spears' mouth was moving but the audience could hear Sia's version of the track.
A video of the performance was posted to Instagram.
Sia is best known for her hit "Chandelier" and is one of the writers of "Perfume," according to Billboard.com. Spears was probably using the songwriter's track as an in-monitor guide but it was mistakenly put into the house mix for everyone to hear.
Britney's show at Planet Hollywood in Las Vegas opened December 2013.
Spears has not commented on the situation.