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Ke$ha has written a song for Britney Spears
The track is called 'Till The World Ends' and is produced by Dr Luke.
Ke$ha told Spin: 'That song is me imagining her and any female musician touring the world.
'You know when you go out and you're having an amazing, magical night and you don't want to go to sleep and you want it to last until the world ends.'
It's not known if the track will end up making the final tracklisting of 'Femme Fatale'.