Prince wrote a song about a postman stealing his girlfriend.

The 56-year-old singer-songwriter - who released albums 'Plectrumelectrum' and 'Art Official Age' last year - is known for having hours of unreleased material, and one of his long-term collaborators, Cat Glover, has revealed one of the unheard songs tells the tale of his lover's affair with a delivery man.

She explained: ''One of the lyrics in his song said, 'I went out of town for 40 days, she made love to the mail man when I was away. I bet she thought I wouldn't catch her. She had a 15 cent postage stamp on her ass.'''

And while the subject matter is slightly odd, the singer - who worked with the 'Purple Rain' hitmaker on his 1987 album 'Sign o' the Times' and 1988's 'Lovesexy' - insisted it is one of her favourite tracks from the star, despite the fact it has never been heard by his fans.

Speaking in the BBC Radio 4 documentary 'Hunting for Prince's Vault', Cat said: ''That's the one I really like.

''That song has never, ever, ever been performed live.

''That is a bad song, and by that I mean awesome.''