CBBC star Archie Lyndhurst, the son of Only Fools and Horses actor Nicholas Lyndhurst, died in his sleep from a brain haemorrhage, his mother has said.
Lucy Lyndhurst said a second post-mortem exam had revealed his death was caused by a condition called Acute Lymphoblastic Lymphoma/Leukaemia.
She described Archie as "the most magical human being we have ever met".
The 19-year-old's death on 22 September had had a "catastrophic effect" on their family, she wrote on Instagram.
Lucy said she and husband Nicholas were assured by the doctor who explained the post-mortem results to them that there "wasn't anything anyone could have done as Archie showed no signs of illness". She said it was "not leukaemia as we know it" and that acute in medical terms meant "rapid".
The couple were "utterly floored" to think something like this could happen, she wrote, adding: "It's very rare and around only 800 people a year die from it."
She said that just days earlier he had been celebrating his birthday with "the love of his life Nethra".
"Life is fragile, precious and sometimes incredibly cruel," Lucy wrote.
She also criticised some media outlets for attempting to garner information about how her son had died from the coroner, before they knew the results of the post mortem themselves.
"To have a coroner call you a few days after your child has died to say the press have been calling for the results of Archie's post mortem, I think stoops to an all time low for us," she noted.
"What gives the press the right to badger a coroner's office solely to find the cause of death before the parents? The complete lack of empathy is astounding. We released no information at the time as we had no idea what he had died from."
Archie began his acting career at the Sylvia Young Theatre School at the age of 10 and was best known for playing Ollie Coulton in the CBBC comedy show So Awkward.
He appeared in the sitcom, which followed the lives of a group of friends in secondary school, from its first series in 2015.
Nicholas appeared alongside his son in a 2019 episode of the programme.
Archie's other roles included recurring appearances as a younger incarnation of comedian Jack Whitehall in various TV programmes.
These included BBC Three sitcom Bad Education, in which he was seen as a younger version of Whitehall's Alfie Wickers character.