'If I could talk to my mum again I'd tell her I had a daughter - and I wrote some books and guess what happened?'

"There was a point where I really felt I had 'penniless divorcee lone parent' tattooed on my head," remarks JK Rowling sardonically. "You couldn't read about Harry Potter without seeing that somewhere in the piece.

"So I thought, 'Fine, let's take that and use it.'"

"Using it" meant sacrificing the relative anonymity that Jo Rowling still enjoyed before she decided last year to lend her support to the National Council for One-Parent Families. The origin of "JK" goes back to the advice from her publisher Bloomsbury that the adventures of an 11-year-old boy who goes to wizard school would not appeal to 11-year-old boys if written too identifiably by a woman.

Ironically, the initials provided a kind of anonymity and mystique, behind which Rowling was able to guard her privacy. Beyond the fact that she was a young-ish single mother who had written parts of the first book sitting in a cafe in Edinburgh because her flat was unheated, relatively little was known. Only when the third book (Harry Potter and the Prisoner of Azkaban) was published did the media's curiosity force her hand: "So I decided that I would give interviews - when I had something to say."

She has plenty to say about the way single parents get blamed for everything from falling morals to rising crime: "It's this universal human desire we have through history: if we demonise them, we don't have to help them. It's much easier for certain sections of society to say, 'You've brought this on yourself by your fecklessness; you sort it out,' than to say, 'You've been a victim of circumstances,' or 'Hey, marriages break up ... but how are we going to help you help yourself?'

"I never set out to be a lone parent - and there I was," she says. "It's undeniable: there's a stigma attached. But I was the most unashamed lone parent you were ever going to meet. I was, like, 'And what is your problem? I'm doing a great job.' I'm very impatient with the idea that any of us should be ashamed about it."

Rowling became a single parent when her marriage to Portuguese television journalist Jorge Arantes fell apart shortly after the birth of her daughter Jessica. "It was my Pamela Anderson and Tommy Lee moment," she jokes. "It is the most humiliating thing, actually: you're supposed to have your relationship disaster in your teens, aren't you - and then wise up?"

Rowling left Portugal in early 1994 and came to Edinburgh, planning to stay with her sister Di for a few weeks until she found her feet. She never left. "Edinburgh is beautiful," she explains, "has good public transport and did have, then, free museums, and I thought, 'I'll have a much better life here on a low income with my daughter.' I could just see that broke single-parenthood here would be easier."

"Poverty is the issue," she continues. "I do say that partly out of my own experience, but I am acutely aware when I talk about single parents being poor that, for me, it was a temporary state and it was always likely to be. We were never going to be rich if I had been a teacher, but we were never going to be dirt poor. And I had middle-class friends who could lend me a tenner."

Rowling's success as an author - the four Harry Potter books to date have sold more than 90m copies worldwide and been translated into 42 languages - has clearly removed that problem. The days of sitting in a cafe, snatching a few hours to write while Jessica slept in her pushchair, are long gone. Jessica, now seven, goes to school, and has a nanny who picks her up two days a week, while Rowling looks after her the other three.

"She's embarrassingly precocious and gloats about the places she's been," she says. "I heard her saying: 'I went to the Space Needle in Seattle, but Mummy couldn't go; Mummy was in a bookshop.' I'm like: 'Yeah, cheers, I know.'"

So does money solve all the problems of being a single parent? "No, but it sure as hell helps. The worst of being a single parent is constantly thinking, 'I can't give you what I want to give you.'"

Rowling named her daughter after Jessica Mitford, sister of Nancy, author and avowed communist, whose autobiography Rowling read as a teenager. "Basically, [Mitford] had every single possible component you could want as a 13-year-old leftwinger."

Rowling looks an unlikely rebel. She was head girl of her comprehensive school in Chepstow - but she was the sort, she says, who got "caught smoking at bus stops with boys in leather jackets". Now 35, she is slim and chic in fitted black rollneck and charcoal hipster pants that flare from the knee. She shakes her head now and then to clear the blonde fringe from her vision - a mannerism that gives her an air of diffidence. But when she fixes you with her blue-grey eyes, you don't doubt that here is a person with some steel.

"It's a funny thing," she muses, "but I doubt very much there would have been a Jessie if mum hadn't died when she did. I only went abroad nine months after she died: I really needed to get away. A lot of bad stuff had happened, and I needed to sort myself out a bit. And obviously, I met my ex-husband and Jessica was the result, so life does go on.

"It was doing a big rebound job, really," she adds. "That's when you're most vulnerable."

Her mother Anne had died in 1990 from multiple sclerosis at the age of 45. With just 20 years between them, Jo and her mother had been very close. "Because I was the older child, to me she was almost like an older sister. I was never in any doubt that she was my mum, but that kind of relationship was there. I could talk to her a lot more freely than some of my friends spoke to their mothers."

Anne Rowling had been 35 when first diagnosed with the illness - "the same age as me now, which is a bit of a facer, isn't it?" Because of the experience of losing her mother, Rowling has recently embraced the MS Society in Scotland and will be helping to publicise MS awareness week shortly. Anne had been a wheelchair user since Jo graduated from Exeter University, but the end, when it came, still seemed sudden.

"The last time I saw her before she died, I went back for Christmas," she recalls, haltingly. "I can't believe in retrospect that I didn't really realise what was about to happen because she was so ill. Her mobility was very limited; she looked ill, very ill - which I'd never really seen before. She was absolutely exhausted.

"She died at home, of respiratory failure. Dad went upstairs and found her dead."

Part of the appeal of the Harry Potter stories is the way Rowling's fiction engages with the dark stuff. Rowling began work on the first book just six months before her mother died. It is one of those well-known facts that the idea of Harry's world first came to Rowling on a train journey to King's Cross; but it was also on a train from that station that her mother and father first met. "That's obviously why it looms large in my psyche," she comments wryly.

Her mother's death, she says, made the fact that Harry is an orphan and the theme of death and loss "immeasurably more important", and she alludes to the now famous episode in chapter 12 of Harry Potter and the Philosopher's Stone, when Harry sees his dead parents in the Mirror of Erised (Desire backwards). What, I wonder, would Rowling see in the mirror?

"It would be my mother," she says without hesitation. "You'd want five minutes to say, 'I have a daughter, and she's called Jessica, and she looks like this and she likes this, and I wrote some books - and Mum, guess what happened?'

"I'd gabble on and at the end of five minutes I'd realise I hadn't asked what it's like to be dead. It's the selfishness of the child, isn't it? - at least I'm aware of that. But it couldn't be long enough. That was all in that mirror episode, I think: recognising that it's just not healthy, not good for you to dwell and dwell and dwell.

"It's not about forgetting, but you have got to move on."

This is an edited version of an article that appears in the May issue of In Style, on sale from Friday.