Did Britney Spears lip-sync to Sia's vocals?

She's no stranger to lip-synching controversies but this time Britney Spears has been accused of lip-synching to Sia's vocals.

A video from Spears' Vegas residency has been posted by a fan to Instagram.

On the video Sia's distinctive vocals can be heard clearly singing the song Perfume, while Spears can be heard faintly singing along.

The prolific Aussie who co-wrote the song along with Spears, provided back-up vocals for the pop star on an acoustic mix of the song and sings a chorus without Spears on the re-release.

The video was posted to an Instagram page set up by a fan called britneysite.

"The 32-year-old pop star struts around and holds her microphone, seemingly singing sweet ballad Perfume - but Sia's voice is booming through the theater!" Radar Online reported.

Spears has been called out for lip-synching on numerous occasions during her Las Vegas residency show, with fans posting videos to social media sites supposedly showing the singer miming.

"You can hear her and sia's voice lmfao dying," one fan commented on this recent Instagram video.

However, this time, some fans are offering alternate views relating to the vocal mix.

One theory for this particular gaffe is that Sia's backing vocals on the track were played louder than Spears' lead vocals during the live show.

"Sia is the back up singer in the chorus. What's the big deal?" one person commented.

Another fan on Instagram had a theory that Spears uses Sia's track just as an in-monitor guide when she sings but it was mixed in, accidentally, to the live show.

"Only this time it (the guide track) was put into the house mix, making for an embarassing accident," they commented.

The 32-year-old just recently returned to her Piece Of Me show at Planet Hollywood in Las Vegas following a brief break. The singer began her residency there in December 2013.

Spears seemed pleased with how her return performance had gone down, tweeting: "Incredible night getting to do what I love! Thx to everyone at the show tonight. Hope y'all had as much fun as I did!"

-AAP