Their relationship has not been particularly well-publicised in the annals of music history, but as unllikely as it seems, pop queen Madonna once went out with rapper Tupac Shakur.

Indeed, Madge only made the revelation in 2015 - but according to a letter written to her by the late hip-hop icon while he was in prison, it sounds like it was pretty serious.

What's more, the letter reveals that the reason for their relationship ending was not down to any romantic reason - but related to race.

TMZ published portions of the previously-unseen letter that Shakur addressed to 'M' while he was incarcerated at the Clinton Correctional Facility, serving time for sexual abuse charges, just over a year before he was shot dead.

It's dated 'Jan 15, 1995, 4:30am' and sees the rapper apologise several times to Madonna, essentially revealing that it would be damaging for him to be seen to date a white woman.

“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,” he wrote. “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.”

At one point, he added that he had been angered by an interview that she had given where she allegedly said: "I’m off to rehabilitate all the rappers & basketball players.’”

It seems that he said some words he regretted, which he apologised for: “Those words cut me deep seeing how I had never known you to be with any rappers besides myself. It was at this moment out of hurt and a natural instinct to strike back and defend my heart and ego that I said a lot of things. …Can you feel me?” In the time since, as you can see, I have grown both spiritually and mentally. It no longer matters how I’m perceived. Please understand my previous position as that of a young man with limited experience with an extremely famous sex symbol.

The letter ended with him extending the hand of friendship, and encouraged her to visit him in prison. "I offer my friendship once again, this time stronger and focused. If you are still interested I would like to further discuss this with you but some of it couldn’t wait. I felt compelled to tell you…just in case anything happened to me,” he wrote. “Please be careful Madonna. Everyone is not as honorable as they seem, there are those whose hearts bleed with envy & evil. They would not hesitate to do you harm! Let my 6 bullets be proof of that!”

The letter is up for auction with a starting bid of $100,000.