The Queen frontman had a child in 1976 after a fling with the wife of a close friend and her existence was kept a closely guarded secret among his inner circle.
In a new book, Love, Freddie, about the late British star - who died in 1991 aged 45 - his daughter, referred to only as 'B' has spoken out for the first time.
Before his death, Mercury handed over 17 of his journals to his daughter, who has shared them with biographer Lesley-Ann Jones for her new tome.
According to The Daily Mail, a handwritten letter from B is included in the book, which reads: "Freddie Mercury was and is my father. We had a very close and loving relationship from the moment I was born and throughout the final 15 years of his life.
"He adored me and was devoted to me. The circumstances of my birth may seem, by most people's standards, unusual and even outrageous.
"That should come as no surprise. It never detracted from his commitment to love and look after me.
"He cherished me like a treasured possession.
"After more than three decades of lies, speculation and distortion, it is time to let Freddie speak.
"Those who have been aware of my existence kept his greatest secret out of loyalty to Freddie. That I choose to reveal myself in my own midlife is my decision and mine alone."
Mercury's daughter is now 48 and lives in Europe where she works as a medical professional.
Jones told the Daily Mail that when she was first contacted by 'B': "My instinct was to doubt everything, but I am absolutely sure she is not a fantasist.
"His only child was conceived accidentally with the wife of one of his closest friends, while his friend was away on an extended business trip. For the Roman Catholic mother, abortion was out of the question," she said.
It was agreed that 'B' would be raised by her mother and her husband, with Mercury having his own room at their house so that he could remain in contact with his daughter.
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