Tupac Shakur broke off his relationship with Madonna because she was white.

That's the shocking revelation of an apologetic letter the hip-hop star wrote to the singer from prison. A letter that comes to Gotta Have Rock and Roll's July 28-ending auction with a hugely conservative $100,000 estimate.

Shakur's letter urges his ex-girlfriend to be careful

The three page handwritten letter, which Shakur wrote in January 1995 while serving time for sexual assault, reads, in part:

"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. I never meant to hurt you."

The pair had an 18-month relationship after meeting in 1993.

Yet this letter suggests Shakur believed his relationship with a white woman was proving detrimental to his image. It's not known if Madonna ever received the letter.

Shakur's letter ends on a portentous note.

"Please be careful," he urges. "There are those whose hearts bleed with envy and evil. They would not hesitate to do you harm! Let my 5 bullets be proof of that!"

Shakur would be killed in a drive-by shooting a year later.

Shakur's personal items have huge appeal with collectors, such is his influence on hip-hop.

An essay Shakur wrote from prison auctioned for $172,275 last year – a record for hip-hop memorabilia. We're anticipating this letter to beat that record – such is its fascinating content and famous recipient.

A sexually explicit letter Shakur wrote to an unknown woman achieved $34,300 in January, while a ticket to the boxing match he attended the night he was murdered in Las Vegas sold for $24,500 at the same auction.

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