Watch: Britney Spears' lip-synching fail… to Sia's vocals!

Oops she did it again…

Britney Spears has been called out yet another time for her shoddy lip-synching but this time it's super awkward.

Fans were quick to point out that Britney's main vocals to 'Perfume' didn't sound entirely like her… but just like 39-year-old Aussie singer Sia Furler.

Judge for yourself in the Instagram video above!

Other supporters argued they could hear remnants of Brit's voice in the background, but the prevalent vocal was definitely Sia's acoustic cover of the pop track. Awks!

"You can hear her and Sia's voice lmfao dying," one fan wrote alongside the Insta-vid.

Another user added: "If you listen closely to the performance video, you can make out some of Spears' vocals under Sia's more charactered voice.

"That's because, most likely, Spears uses Sia's track as an in-monitor guide when she sings. Only this time it was put into the house mix, making for an embarrassing accident."

Embarrassing indeed! But this isn't the first time Brits has been caught out for a lip-synching fail at her current Vegas residency.

But let's not forget her worst yet, which was at the 2007 Video Music Awards. Check it below!

Her manager Adam Leber responded back in June to the many fan accusations of Britney's miming.

"To put on the show that she puts on, it's virtually impossible to sing the entire time and do what she does," he told Medium.com. "She's singing on every song, basically, when she has the ability to sing. There's no way you can dance for 90 minutes straight and sing the entire time."

Want to see more lip-sync fails? View the worst of the worst...