A source told Britain's The Sun newspaper: "Madonna is always trying to raise the bar and Britney and Justin on the same stage would be perfect." Madonna is said to be hoping to convince Britney and Justin - who split in 2002 after four years together - to appear with her on the final US date of the tour in Miami, in November.

The source added: "They have both been asked but I think they are both a bit reluctant. Britney would find it all a bit emotional and Justin might not be interested. More likely is that they will both appear at different dates.

But if anyone can make it happen it's Madonna." Britney, 26, has already recorded a video segment to be screened on the tour. Filmed in a lift, it shows her in a white vest getting hot and sweaty as she kicks and punches the walls.

Madonna has also asked Britney to appear live with her as a surprise guest at some dates on the tour. Justin, 27, was heavily involved with the making of Madonna's latest album 'Hard Candy', and Madonna has also asked him to duet with her during the tour.

Justin appears on her worldwide hit single '4 Minutes' from the new album and has writing credits on five of its tracks. Madonna's 'Sticky and Sweet' world tour kicks off in Cardiff, Wales, on August 23, and takes in Europe, North America and Mexico.