The pop veteran is said to be joining the 'Womanizer' singer as a special guest, after Britney performed with Madonna in Los Angeles last November.

"When Britney joined Madonna on stage for her Sticky + Sweet tour it was very sedate." a source tells British newspaper the Daily Mail. [a source? Maybe the concession stand attendant?] "This is going to be a high-energy dance routine - getting back to what they are both good at: Performing." [we could go to town on that last bit but we'll leave it up to your imagination] - more on this story

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