The "Toxic" singer, who was due to appear at Los Angeles' Greek Theater on Saturday (June 30, 2007), failed to accept the offer to join the "True Colors" tour.

A representative for the tour said: "She was invited to appear on the first date in Las Vegas but we never heard from her."

Britney reportedly decided not to appear because her recent comeback performances have been dogged by lip-synching rumors and she was said to be terrified of singing live.

A source told the New York Post newspaper: "Britney said she would only dance or lip-sync - and to be on stage with Cindy, you have to actually perform. It's not happening now."

Earlier this week, Britney's choreographer Misha Gabriel revealed the 25-year-old was planning to join Lauper, Debbie Harry, Erasure, The Dresden Dolls, and The MisShapes on stage this weekend.

Rosie O'Donnell, who is set to present the show, admitted she was excited about meeting Britney backstage before the performance.

Rosie wrote on her blog: "Brit, honey, you and me - we're gonna share, like, a backstage moment, 'cause I love you, kid!"

The "True Colors" tour raises funds for gay, lesbian, bisexual and transgender organizations.