Archie Lyndhurst, the son of Only Fools And Horses actor Nicholas Lyndhurst, died from a brain haemorrhage, his mother has confirmed.
The 19-year-old died in his sleep at his home in Fulham, west London, on 22 September.
Lucy Lyndhurst shared the results of her son's second post-mortem on Instagram, writing: "He died from an Intracerebral Haemorrhage caused by Acute Lymphoblastic Lymphoma/Leukaemia."
Calling the details of the report "harrowing", she said it was "not leukaemia as we know it" and that in medical terms, acute meant "rapid".
An intracerebral haemorrhage is caused by bleeding on the brain, causing a stroke.
She also explained that the doctor had found "numerous bleeds on the brain" and that "Archie wouldn't have been in any pain as it happened in his sleep".
As Archie had shown no signs of illness, and his death was due to natural causes, she said there was nothing that could have been done to prevent his death.
Early reports following his "unexplained" death said he had died following "a short illness".
His mother described the effect of Archie's death on the family as "catastrophic", calling him "an extraordinary magical human being", "an old soul" and "incredibly advanced for his years".
An only child, she said that she and Nicholas Lyndhurst were "grateful and privileged to have been chosen to be his parents".
Archie's father famously played Rodney Trotter, the hapless younger brother of Sir David Jason's Del Boy, in Only Fools and Horses.
Archie had been dating his So Awkward co-star Nethra Tilakumara, whom his mother called "the love of his life", and had celebrated her birthday with her just days before his death.
Lucy also shared a series of photos of Archie, showing him as a child, and later as a successful actor, as well as relaxing with his girlfriend.
Calling life "fragile, precious and sometimes incredibly cruel", his mother said "to lose a child is every parent's nightmare", adding that she "wouldn't wish this pain on anyone".
She signed off her post: "Our darling boy, we love you forever and ever and thank you every day, for every beautiful memory we have. We will celebrate you always. All our love. Mama and O.M."
Archie's funeral took place in a "tiny chapel" in November, his mother confirmed in an earlier message.
Archie trained at the Sylvia Young theatre school, and was best known for playing Ollie Coulton in CBBC comedy So Awkward.
He had also appeared in hospital drama Casualty and in Jack Whitehall comedy Bad Education.