Her Planet Hollywood residency in Las Vegas is called Piece of Me, but it became evident last night that it wasn’t a piece of Britney that was being offered up to fans, but rather a piece of Sia, the singer/songwriter behind a number of tracks on Spears’ latest album.

In a mix-up that will no doubt cause someone to be fired – and we doubt it will be Spears – the singer was left bizarrely lip-syncing to a track that contained a very prominent vocal, the distinctive tones of Sia, the singer/songwriter who wrote the track ‘Perfume’ for Spears Britney Jean record.

A fan captured the video and posted it to Instagram, which you can see below. Compare it to a previously remixed clip of the track, which features the vocals of both Spears and Sia.

Britney has been plagued with complaints about her lack of live singing throughout her career on each of her concert tours, though the star’s manager Larry Rudolph told fans and media that the 32 year-old would “definitely not be lip-syncing across the board” when her Planet Hollywood residency was announced last December.

The singer confirmed that the show would contain live vocals in an interview with Chicago 103.5 KIIS FM, saying “I’m definitely going to be singing live. I always sing live. It doesn’t sound so great all the time, but I do my best”.

Despite the lack of live vocals, Spears’ residency has so far received mostly positive reviews from media, though its supporting record Britney Jean, rush-released to coincide with her Vegas venture, has suffered heavy criticism from both fans and press.

The album’s opening track ‘Alien’, produced by William Orbit, featured an obvious production glitch, while several tracks were said to contain lead vocals by Spears’ longterm backing singer Myah Marie, who in the past has told of her ability to mimic ‘Britney voice’ on command.