Nearly a dozen passionate letters sent by Mick Jagger to his secret lover while he was in Australia in the summer of 1969 have sold for STG187,250($288,587).
The price was more than double the pre-sale estimate of STG70,000 to STG100,000.
They were bought by a private collector, bidding over the telephone.
The letters, said to paint a picture of Jagger as a "poetic and self-aware" 25-year-old, were written to American-born singer Marsha Hunt while the Rolling Stones frontman was in Australia.
Hunt, who provided the inspiration for the Stones' 1971 hit Brown Sugar, had tasked Sotheby's in London with selling the 10 letters at auction.
At the time she was the image of Black is Beautiful and the face of the landmark West End production of Hair.
After the sale, Hunt said: "The passage of time has given these letters a place in our cultural history.
"1969 saw the ebbing of a crucial, revolutionary era, highly influenced by such artists as The Beatles, The Rolling Stones, James Brown and Bob Dylan.
"Their inner thoughts should not be the property of only their families, but the public at large, to reveal who these influential artists were - not as commercial images, but their private selves."
Dr Gabriel Heaton, Sotheby's books specialist said: "We are delighted with the result of today's sale which reflects the great significance of these letters, written at such a vivid moment in social and musical history.
"There has been enormous international interest in the letters, which depict Mick Jagger, not as the global superstar he is today, but reveal him as a poetic and self-aware 25-year-old with wide-ranging intellectual and artistic interests."
The collection, which included song lyrics and a Rolling Stones playlist, was the centrepiece of an English literature and history sale.
Written from a film set in the outback just after the Stones' landmark Hyde Park concert, the documents provide an insight into the cultural events of the time, including the first moon landing.
Dr Heaton said: "They provide a rare glimpse of Jagger that is very different from his public persona: passionate but self-contained, lyrical but with a strong sense of irony."
The letters were penned by Jagger for his lover while he was filming the movie Ned Kelly and their relationship was a closely guarded secret.
Hunt said of the well-preserved letters: "They're addressed to me.
"I was 23, American-born, Berkeley-educated and London-based.
"Despite his high profile and my own... our delicate love affair remains as much part of his secret history as his concerns over the death of Brian Jones and the suicide attempt of his girlfriend, Marianne Faithfull."
In an interview with The Guardian, Hunt, the mother of Jagger's first child, said she needed to raise funds to repair her house in France.
Now aged 66, the singer told the newspaper she hoped the buyer would recognise that they have a piece of history.