Madonna described Whitney Houston and Sharon Stone as ''horribly mediocre'' in a letter she penned in the 90s.

The 58-year-old icon detailed her bitterness towards the duo in a note that Darlene Lutz, Madonna's former art curator, whom she sued in 2005 for breach of contract, has provided to the auction site

In the letter, the 'Like a Virgin' hitmaker said: ''It's so unequivocally frustrating to read that Whitney Houston has the music career I wish I had and Sharon Stone has the film career I'll never have.

''Not because I want to be these women - because I'd rather die, but they're so horribly mediocre and they're always being held up as paragons of virtue [or] some sort of measuring stick to humiliate me. (sic)''

The letter, which is poised to go under the hammer next week, is expected to sell for between $3,000 and $5,000, according to the New York Post newspaper.

Meanwhile, it was recently revealed that rap star Tupac Shakur dumped Madonna because she's white.

The late musician - who died in September 1996, after being injured in a drive-by shooting - dated the chart-topping singer for three years before his death.

But in a gripping letter Tupac penned when he was in prison, the rapper revealed that he ended their secret relationship because of their different cultures and how his ''people'' would react.

He wrote at the time: ''For you to be seen with a black man wouldn't in any way jeopardize your career. If anything it would make you seem that much more open and exciting. But for me at least in my previous perception I felt due to my 'image' I would be letting down half of the people who made me what I thought I was. (sic)''