The band - who are set to perform before Muse on the Pyramid Stage at the world famous music event - have reportedly lined up an appearance by the 'Lord of the Rings' actor when they perform new track 'Invisible Light', from their upcoming album `Night Work', on which he feature on vocals.
Singer Ana Matronic told The Sun newspaper: "I would die to have him up there. I met him at Live Aid. He was backstage hanging out, so I think he's a good time Charlie. He could get into it."
She went on to joke that she wants McKellen - who played wizard Gandalf in the 'Lord of the Rings' trilogy - to wear a rubber wizard costume while performing with the group.
The Scissor Sisters are not the only band performing at Glastonbury who hope to be joined on stage by a guest star.
Headliners Muse have promised a "very, very special surprise guest" will perform alongside them.
Drummer Dominic Howard revealed: "If it comes off, it'll be a huge moment for Glastonbury. It will take it to a whole new level for us."