On Mondays, Wednesdays and Fridays it was Court Hand and Summulae Logicales, while the rest of the week it was the Organon, Repetition and Astrology. The governess was always getting muddled with her astrolabe, and when she got specially muddled she would take it out of [sic] the Wart by rapping his knuckles.

So begins The Sword in the Stone, TH White's magnificent 1939 retelling of the Arthurian legend, which is followed by three more volumes that together make up The Once and Future King. As a boy, I was immediately pulled in to this world both for its familiarity (what child's week is not regulated beyond their control?) and for its strangeness. What, exactly, were Summulae Logicales and the Organon? The magical thing was that it didn't matter: I was with Wart, entirely, a small flame sparked within my heart.

We meet Wart – called so by his foster-brother Kay "because it more or less rhymed with Art, which was short for his real name" – when he is in a position of weakness. He is largely pushed about by those in charge of him, whether it's the Sergeant-at-Arms, his nurse, or his kindly foster-father Sir Ector. Long before Harry Potter, here was a boy who finds respite in magic. The distinction is that what he experiences is, unlike Potter, geared to teaching him how to behave in the real world.

This is true magic: turning Wart into a fish, where he learns about the wrong sort of power from a pike; sending him amongst the horrible, bellicose ants or the calm, simple geese; or even simply to quest in the ancient greenwood with Robin and Marian. It is magic as extended metaphor: poignant, beautiful and clear.

Wart is the archetypal child that has power thrust upon him thanks to the workings of an implacable fate. He is by nature kind, generous and courteous, and very much a human (when Kay speaks contemptuously to him, he rages like anyone else would.) Wart, of the hay-fields, the forests and the laundry room, is an ordinary boy, but what he comes to do, as King Arthur of the glittering towers and fluttering banners of Camelot, is beyond extraordinary. TH White, through Wart, shows that all children have the capacity within them to overcome not only outside malevolence, but also the evils that threaten from within.

The Sword in the Stone begins with Wart on a "quest" to find a tutor. Questing is inherent to the Arthurian cycle. I like to think of Childe Roland, the paladin whose journey to the Dark Tower forms the basis of my new book The Broken King, as on the fringes of the Arthurian court: perhaps he pricked past Arthur on the plain, had a friendly joust, and galloped off again, his helm glinting in the sunlight. Wart, as he becomes Arthur, has the hardest quest of all: to hold his kingdom together, in the face of both temporal and spiritual attacks, his court dissolved, his army defeated, his death approaching. There is a reason why Malory called his romance Le Mort d'Arthur.

The Once and Future King is about history and the passing of time. One of the most touching moments is right at the beginning, when Wart meets Merlin for the first time – only for Merlin, who lives backwards, it's the last time. The old wizard weeps and the boy can't understand why. It's a powerful expression of the gulf between the ancient and the young.

This is displayed on a grander scale: White manages to compress several centuries of British history into his work, and Wart sees it all, remaining sympathetic, valorous and humanly flawed till his final, tragic (in the Aristotelian sense, as White himself would have it) end. Wart oversees the transformation of Britain from a world where Might is Right, through his own more civilised court, to the decadent cynicism of Mordred and his twisted views. Wart can understand what goes before him; but King Arthur can't change what's going to happen. Such is the pain of human existence.

One of the last things that Wart does before his death is speak to a little page, Thomas. "My idea of those knights," he says of the Round Table, "was a sort of candle... I have carried it for many years with a hand to shield it from the wind. It has flickered often. I am giving you the candle now – you won't let it out?"

Thomas, of course, is Thomas Malory, and he obeyed his king, and carried that candle. The spark, first lit by a small boy in the gloom of an ancient castle, now shines brightly throughout the world, for freedom, honour and all that is good: and thanks in part to Wart's gentle majesty, it will never go out.

Philip Womack's latest book, The Broken King is available from the Guardian Bookshop.