Spears lip-synched and danced to her new single, "Womanizer," as a celebrity guest performer on Saturday night's show and earned a standing ovation by "The X Factor" judges. However, her eagerly awaited performance was criticized on show message boards by fans after the Spanish-born singer Ruth Lorenzo was booted from the competition.

"It was terrible," The Times said one viewer wrote. "All the contestants are better than her."

Another armchair critic griped, "She wasn't even trying to sing."

But the show's producers maintain they were happy with the performance.

"We were absolutely delighted at having the American pop phenomenon that is Britney Spears appearing on the No. 1 entertainment show in the United Kingdom," executive producer Richard Holloway told The Times.

A spokesman for the show added that Spears's performance was "brilliant."

"What we promised and what she wanted to deliver was a performance where she was back to her best, and that was what she did. She's always been known for her dancing. We're surprised at people's reaction," a source close to Spears told the newspaper.