It is a story that is already worth more than $100 million of investment to film-makers at Warner Brothers, with much more to come. It has made its once impoverished author a millionaire many times over, while its enormous popularity in 200 countries and 47 languages is self-evident.

Now a child psychotherapist is to reveal exactly how J.K. Rowling did it. A theory to be published next month will aim for the first time to explain the success of Harry Potter in psychological terms.

Margaret Rustin, a distinguished psychotherapist at London's Tavistock Clinic, has isolated the factors in Rowling's work that ensure its appeal for adults and children.

This story of wizards, potions and broomsticks, it turns out, in fact deals with archetypal fears of loss and separation and, as such, is copying a formula set down by other great works of children's literature, such as C.S. Lewis's The Lion, the Witch and the Wardrobe.

'What struck me very strongly,' said Rustin, 'is that Rowling starts with a child who has lost his parents. All her stories, from The Philosopher's Stone onwards, are to do with the emotional rediscovery of his parents, in his own mind, even if not in reality.'

Rustin, who studied Rowling's work with her sociologist husband, Michael, has worked with children at the Tavistock Clinic, featured in the BBC2 documentary Talking Cure, since 1971 and she believes that literature can reach out to children.

'If you think of other stories, such as Charlotte's Web by E.B. White or The Lion, the Witch and the Wardrobe, they take place in the context of parents being absent. This is how the child learns to cope with separation and loss.'

In the Harry Potter stories, Rustin believes that the teachers at Hogwart's School each represent a different aspect of parenthood.

'Dumbledore is in some ways the ideal father figure, while Minerva McGonagall stands for the the ideal mother,' she says.

The Rustins' analysis of the Potter stories is to appear in a paperback re-issue of their 1987 Verso study of children's literature, Narratives of Love and Loss, published by Karnac Books.