The 'Creep' hitmakers are expected to make an appearance on the park stage of the three-day festival in Somerset, South West England, although organiser Emily Eavis is afraid to confirm their set because of overcrowding fears.
She said: "We do have a special guest that's a really big thing for us. It's a band who could potentially be headlining.
"I don't want to big it up too much or we might have to close the field."
A source told the Daily Mirror newspaper: "Originally the plan was simply for Kelly and Michelle to head down to Somerset to see their old bandmate.
"Beyonce was over the moon and sorted them out with Access All Areas passes and accommodation. From there plans developed and the idea of a brief on-stage Sunday evening reunion was mooted.
"One idea is for the two girls to join her on stage towards the end of her set for a medley of their bits. But whatever happens, the three girls are getting back together at Glastonbury and plan on having an absolute ball."