Britney Spears is under fire yet again for using backing vocals by Sia during her Las Vegas residency.

The Womanizer hitmaker is currently in the middle of her two-year Britney: Piece of Me stint at the Planet Hollywood Resort and Casino, and during a recent performance, a fan recorded her singing a new version of her song Perfume, and posted a clip on Instagram.com.

However, Billboard.com points out that it was songwriter Sia's vocals the crowd mostly heard, and not Spears'.

While Sia is one of the co-writers on the track, a few non-official demo versions of the Chandelier hitmaker singing Perfume can be found online, and her vocals are the ones used by Spears to lip sync over in her Sin City show.

Editors of the music site suggest Spears uses Sia's track as an in-monitor guide when she sings, however the backing vocals were accidentally used as the house mix which the audience heard.

The gaffe comes weeks after Spears came under scrutiny when a raw clip of her 2013 song Alien surfaced online, prompting critics to mock the singer's out-of-tune vocals.