Britney Spears is under fire once again for sloppy lip-syncing. She outraged fans over the weekend during her residency performance at the Planet Hollywood Resort and Casino in Las Vegas, after allegedly singing Sia's backup vocals during "Perfume."

Spears' rep told Mail Online that was "not true" and that it was her own.

Sia co-wrote the track "Perfume" and was featured on an acoustic version, but the rep notes this was not the one Sia was featured on. Fans believed it was Sia's voice coming out of the speakers, not Spears.

The "Crossroads" star's manager Adam Leber defended her lip syncing in June to Medium.com.

"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."

Larry Rudolph echoed these claims last September, explaining, "No matter what anyone says, there's not a single artist who goes out there and does full choreography and is singing without a vocal track underneath them at the same time."

"It's physically impossible. We're just trying to build up her stamina and get her to a point where she can do as much as humanly possible. The idea is to try to get her pretty close to 100%."

He continued, "There might be some numbers where she's full out dancing with a (vocal) track underneath her, but there won't be any lip-syncing across the board on anything."

However, Spears did say last year she would sing live for the shows.

"I'm definitely going to be singing live," she told Chicago's 103.5 KISS FM at the time. "I always sing live. It doesn't sound so great all the time, but I do my best."

The "Crazy" singer began her two-year residency in December and will be paid $15 million a year.

Do you think Spears lip synced to Sia's track? Share your thoughts in the comments section below!