Freddie Mercury had a secret daughter, according to a bombshell new biography on the Queen rocker.

Lesley-Ann Jones’ upcoming retrospective on the legendary singer’s life, “Love, Freddie,” includes a handwritten letter detailing the existence of a daughter conceived by Mercury and the wife of a friend in 1976 — at the height of his career.

The guarded secret was known only to the late “Bohemian Rhapsody” star’s nearest and dearest –including his longtime love, Mary Austin, his parents, and his sister — for nearly half a century, Daily Mail reported Friday.

Now 48, the anonymous daughter, identified as “B,” is a mother and a medical professional who resides in Europe.

The letter appears in chapter one of the biography, which is set to be released on Sept. 5.

“Freddie Mercury was and is my father,” she wrote. “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,” she continued. “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.”

“B” was raised by a loving family, though the letter made it clear Mercury visited her often, and that she was aware of who her biological father was.

Prior to his death in 1991, Mercury gave his daughter 17 journals, which have also remained secret through the decades — and were passed to the author ahead of the book’s publication.

Jones, for her part, confessed she initially questioned the claims.

“My instinct was to doubt everything, but I am absolutely sure she is not a fantasist,” she said, according to the outlet. “No one could have faked all this,” Jones said in part.

The author, who has also written biographies on John Lennon and the Rolling Stones, among others, explained why she felt the woman’s claim is legitimate.

“In my experience of fantasists, and I’ve met a few, they seek instant gratification, publicity and reward,” Jones said.

“She has never asked for money. She does not want recognition. Both Freddie and her stepfather left her extremely wealthy. She was not provided for through Freddie’s will, but by a private, legal arrangement, so no one will find her mentioned there.”

She also asserted that Mercury was “not who you think he was.”

“He was a hands-on, devoted dad,” Jones said in part.

The author claimed “B” was conceived “accidentally” with the wife of a close friend while he was away on business for a long stretch.

Given that the mother was Roman Catholic, abortion was “out of the question.”

“It was decided between the three adults that the child would live with her mother and her husband – the child’s stepfather,” the author claimed, with the singer having a room at each of the residences as the trio raised the child.

Jones said Mercury spoke to the girl “every day” while he was away on tour. The author asserted Mercury’s daughter knew “from toddlerhood” which of the two men was her “real father.”

“Outside the unusual family, privacy and discretion were maintained to a degree that not even some members of Freddie’s personal household had any idea that he had a child,” Jones said.

The flamboyant showman, whose last male lover was the late hairdresser Jim Hutton, had a well-known romance with Mary Austin, to whom he was engaged, prior to coming out as gay. In the 1980s, he dated actress Barbara Valentin.

The mother of the secret child was kept under wraps, though she is said to have died “years ago,” according to the outlet.

In another letter in the book, “B” described her father as an “intensely private” man — a quality she claimed to share with him.

“The life I live with my husband and our family in another country is intensely private,” she wrote in part. “We want things to stay that way. We cherish our peaceful and anonymous life, and we want nothing to disturb it. Nobody needs to know who I am.”