A series of "passionate and articulate" letters sent by Mick Jagger to his secret lover, Marsha Hunt in the summer of 1969 are to be auctioned in December.

The letters, which are said to paint a picture of 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.

Mick was filming the movie Ned Kelly, in the Australian outback, whilst corresponding with Hunt and the documents reference landmark events of the time, including the first moon landing.

Auction house Sotheby's will be selling the 10 letters which are expected to fetch between £70,000 and £100,000.

Gabriel Heaton, Sotheby's book 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.

"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."

Marsha, who is said to have provided the inspiration for the Stones' 1971 hit Brown Sugar and is the mother of Jagger's first child, said she was selling the letters to raise funds to repair her house in France.

Speaking about the documents she said: "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."

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."