Polly will play The Pyramid Stage on 25 June.
The singer, who will also play T in the Park and Oxegen Festival has confirmed the Glastonbury appearance on her website.
In other Glasto related news, the music press has speculated this week that Oasis (the worst kept secret since Paul McCartney), OutKast, James Brown and Prince will also be at Worthy Farm in 2004 giving the festival a unusually funky flavour.
Tickets go on sale 1 April at 8pm.
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