(XFM.co.uk) -- U2 have ended months of speculation by ruling themselves out of a headline slot for next year's Glastonbury Festival. But could it be Eminem, Prince, or Madonna instead?

U2 had been hot favourites to fill the coveted Saturday night slot (graced this year by Sir Paul McCartney) ever since festival-founder Michael Eavis told Virtual Festivals of his intentions to book the band back in June.

But last week, Eavis revealed to Virtual Festival that the band have reluctantly declined his offer due to a busy world tour next summer.

"U2 are not doing it. We heard last week,” he explained, “They are doing a tour with Clear Channel and it would get in the way of their schedule. But their manager said they will do it at some point in the future. We’re always talking to potential headliners from a very early date, because they are the most complicated to sort out."

When quizzed on likely contenders for the three headline slots for next summer's event, Michael remained tight-lipped - but did drop a few intriguing clues.

"We know who we want the three headliners to be, but I can’t tell you who!” he continued, “What I will tell you is that we have sent someone out to the States this week to pursue an artist we want. Yes, they are American and yes, they are of superstar proportions. That’s all I can say!"

If by “an artist" he means a solo singer, then Prince has to be hot favourite. The artist formerly known as a squiggle has been coveted by Glasto organisers for some time. Other strong possibilities include Eminem, Madonna or Marilyn Manson.

The three headliners, along with the rest of the line-up for Glastonbury 2005, will not be officially announced until after tickets go on-sale next April.