William Blake - poet, painter, master craftsman, gentle mystic and outspoken radical thinker - was above all a fierce champion of the power of imagination. Born in Soho, central London, on November 28, 1757, he was a startlingly unusual child - at the age of four he saw God look in through the window, and at eight he saw a tree full of angels at Peckham Rye. His father, one biographer has said, “recognised that so strange and stormy a child must be spared the discipline of school”. But Blake himself put it more plainly: “Thank God I never was sent to schoolTo be flog’d into following the life of a fool.”

He learned to read and write at home and, because of his precocious artistic talent, was sent at 10 to study at a drawing school in the Strand, where he drew from casts the torsos of antiquity and developed a lifelong passion for Michelangelo. At 14 he was apprenticed to the engraver James Basire.

This, the first major Blake exhibition since 1978, offers a new generation valuable insight into his work as an apprentice. Much of his time was spent drawing tombs in Westminster Abbey - he was profoundly affected by the artistic integrity of the Gothic world. Although he despised organised religion, the Bible was Blake’s most frequent source of subject matter, and the drawings he made in the abbey are echoed throughout his work. With their draperies and archaic attitudes, his figures have the timeless quality of stone carvings - even when they’re in full flight, like Dante running from the three Beasts, or upside down in a vat of fire, like the Simoniac Pope, they maintain their classical poses, their well-muscled limbs and eloquent, finely drawn hand gestures.

The second section deals with Blake’s life in Lambeth, south London, where he lived and worked with his wife, Catherine. She was illiterate when they married but he taught her to read, write and draw - there’s a drawing by her of the young Blake. She also learned to see visions, as Blake did. It was in Lambeth that the Blakes were famously interrupted naked in their garden, reading Milton’s Paradise Lost.

The third section is the most ravishing and dramatic, exploring the arcane secrets of Blake’s visionary universe, his symbolism and his complicated mythology. Here are the 12 large, colour prints he created by painting his design on millboard, then taking a monoprint and working up the print with ink and watercolour. Chance effects in the printing give these images extraordinary depth and texture as well as a crepuscular luminescence - look at Nebuchadnezzar humbled on hands and knees, lit from within by a fiery glow, or Newton seated under water, the precision of his sleek sinuous limbs and rippling torso in dramatic contrast to the random, swirling bstraction of the rock beneath him.

The final section displays some of the illuminated books - including Songs of Innocence and Experience, Jerusalem and The Song of Los - a fragment of Blake’s subversive “Bible of Hell”, which questioned the authority of the earliest books of the Bible. These are the works by which Blake most wanted to be remembered.

Most of Blake’s pictures, especially those in the illuminated books, are very small. This is a vast exhibition and there’s an inevitable lack of intimacy, but that’s well compensated for by the presence of a huge, wooden rolling press along with engraving tools, inks and so on that gives a tangible idea of the physical - and dirty - nature of the work the Blakes undertook to produce the little books that have such an important place in the history of illustrated literature. In Blake’s time, illustrations were usually etched or engraved with text typeset and printed separately. With the help of his brother Robert (who had died two years earlier, but appeared to him in a vision), he invented a relief-etching technique. It was a delicate, difficult, laborious task, but the results were magical - to see a poem such as “The Tyger” or “The Echoing Green” in its original form is moving and astonishing. Facsimile copper plates show the stages of this process. With text and illustration thus interwoven, the words become part of the overall design, achieving exactly the sort of integration that today’s illustrators of children’s books strive for. Each copy was hand-coloured, and the books were stitched together by Catherine.

In a richly informative introduction to a vast and magnificent book, William Blake: the complete illuminated books, published to coincide with this glorious exhibition, David Bindman quotes from Blake’s wonderfully grandiose announcement in 1793 about this liberating new invention, which allowed him to be author, illustrator and publisher: “The Labours of the Artist, the Poet, the Musician have been proverbially attended by poverty and obscurity; this was never the fault of the public, but was owing to a neglect of means to propagate such works as have wholly absorbed the man of Genius. Even Milton and Shakespeare could not publish their own works.”

It’s easy to see why Blake remains a uniquely inspirational figure. The poets, artists and musicians responding to his work in the Tate’s events programme include Adrian Mitchell, Michael Horowitz, Patti Smith, Sir John Tavener and Alex James of Blur.

William Blake is at Tate Britain until February 11, 2001. School group tickets pound;2 each, free teacher’s pack, pre-visits for teachers at concessionary rate of pound;5. Bookings and details on 0207 887 8888.William Blake: the complete illuminated books, Thames amp; Hudson pound;39.95