Britney Spears is embroiled in another lip-syncing controversy after she was accused of miming during the launch of her Las Vegas residency.

The pop superstar kicked off her Britney: Piece Of Me shows at the Planet Hollywood Resort and Casino in Sin City on Friday night (27Dec13), and concert reviewers have now accused the star of letting down fans by failing to sing live.

A number of critics claim Spears has reneged on a promise to provide live vocals on stage after the singer's manager, Larry Rudolph, previously insisted the show would not be mimed, saying, "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."

Caryn Ganz of Rolling Stone is adamant the Toxic hitmaker was not singing live, writing, "While the show's producers claim Spears is singing along with backing tracks of her own voice, there was little evidence Britney sang a live note."

The Los Angeles Times' Mikael Wood adds, "The singer's presence felt so diminished - her dancing a tentative shadow of what it used to be, her vocals apparently lip-synced for the majority of the show."

Keith Caulfield of Billboard.com adds, "The vast majority of the show was lip-synced, and only occasionally were any seemingly live vocals heard. There was a certain thrill in actually hearing Spears breathing into the microphone - however infrequent."

However, USA Today's Marco della Cava could not decide whether the star was singing or not, writing, "As for the oft-raised specter of lip-syncing, it's impossible to gauge whether there was any funny business with the vocals... These club-oriented tunes all require such different effects that her voice surely had to be run through computers to correspond with the tone of what was recorded in the studio."

Spears was previously accused of miming during shows on her Femme Fatale Tour in 2011.