J.K. Rowling's mother died unaware that her daughter had begun to draft the children's story about a young wizard called Harry Potter that would one day turn her into one of the most famous authors in the world.

In a television documentary to be broadcast tonight, the writer reveals her sadness at the fact her mother had never known she had started a book.

Rowling was only 25 when her mother, Anne, died of multiple sclerosis, aged 45. She had been suffering from an aggressive form of the neurological disease for a decade.

The author made the rare personal statements on a BBC investigation into the prevalence of MS in Scotland. She said: “I started writing Harry six months before she died. That's obviously a real regret, because I never told her I was even writing it. She knew I wanted to write - I'm not sure how seriously she took it. She never knew anything about Harry Potter.”

Rowling also spoke of her pain at witnessing the gradual decline of her mother, whom she had perceived as invulnerable before her death in 1990. “When I left home to go to university, she was walking unaided. By the time I graduated, she was in a wheelchair and in the house she needed a walking frame. It was awful to watch.”

The author said her mother was showing signs of MS years before it was diagnosed. She went to a doctor when she experienced a persistent numbness in her right arm.

Rowling, who in 2006 made a large donation to start a leading research centre into MS in Edinburgh, expressed frustration at the level of funding and awareness of the condition. Scotland has one of the highest prevalences of MS, with one in every 500 people affected. Describing it as a “a Cinderella of illnesses”, she said: “You hear this all the time, because it's underfunded, because it's ignored. I think that it's possibly common to a lot of neurological conditions. It just seems to be an area that has not seemed very sexy for funding. People get diagnosed and sent home.”

The investigation was carried out by Elizabeth Quigley, a BBC journalist who herself suffers from MS. “What she said was very poignant. We all know about Harry Potter - the whole world knows about him - but her mother didn't,” said Ms Quigley. “It is a subject she feels very passionately about. I feel very lucky that she agreed to speak to me in such depth about her mother and what's happening with the disease.”

Ms Quigley, who is married to John Swinney, the Finance Secretary in the Scottish government, said that she made the programme because she wanted to find answers. “Why do we have the highest rate of MS in the world?” she said yesterday. “I'm a journalist with MS and I can't answer it.”

Her investigation took her to Orkney which, along with Shetland, is a hotspot for the disease, and then to Canada, where MS is common.

“The bizarre thing is that there seems to be an Orcadian gene - take that to its logical conclusion and we seem to take it with us, exporting it around the world. It is highest in places where the Scots went, such as Nova Scotia.” Ms Quigley said Scotland should become the epicentre of MS research. “The scientists I spoke to, although they wouldn't give me a timescale, said that finding a cure was a feasible goal.”

Scotland's Hidden Epidemic: the Truth About MS will be broadcast on BBC Scotland at 10.45pm Wednesday, July 23.

Times Recommends

• Brown gambles it all on rescue plan
• Motherhood boosts female brain power
• That's no old FT, that's my naked wife