A series of "passionate and articulate" letters sent by Sir Mick Jagger to his secret lover in the summer of 1969 are to be auctioned next month.
The letters, which are said to paint a picture of Sir Mick 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, has given Sotheby's the commission to sell 10 letters, which go under the hammer in London on December 12.
Predicted to realise between £70,000 and £100,000, the collection - which also includes song lyrics and a Rolling Stones playlist - will be the centrepiece of an English literature and history sale.
Gabriel Heaton, Sotheby's books specialist, said: "These beautifully-written and lyrical letters from the heart of the cultural and social revolution of 1969 frame a vivid moment in cultural history.
"Here we see Mick Jagger not as the global superstar he has become, but as a poetic and self-aware 25-year-old with wide-ranging intellectual and artistic interests."
Written from a film set in the Australian Outback just after the Rolling Stones' landmark Hyde Park concert, the documents provide an insight into the cultural events of the time, including the first Moon landing.
Dr Heaton added: "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 Sir Mick for his lover while he was filming the movie Ned Kelly - he was also involved with Marianne Faithfull at the time and their relationship was a closely-guarded secret.
In a statement released by Sotheby's, 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 Sir Mick'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.
"The letters speak for Mick at an incredible juncture of our lives," she said.
"Someone, I hope, will buy those letters as our generation is dying and with us will go the reality of who we were and what life was."