The 76-year-old "Lord of the Rings" star revealed he enjoyed an innocent holiday romance with a girl named Wendy when he was a kid, years before realising he was gay, and they kept in touch after the vacation by sending one another letters in the mail.
During an appearance on US morning show "The View", he recalled, "I had a straight experience (once)... I must tell you about Wendy... She was nine and I was eight and we held hands by the sea and such, and then we parted after the holiday and we wrote letters to each other."
McKellen reportedly signed a USD 1.6 million deal with publishers Hachette to pen his life story after turning down offers for years.
The yet-to-be-titled book is expected to include details about how he agonised over revealing his homosexuality, amid fears it would damage his career. He eventually 'came out' as a gay man in 1988.