GEORGE Stubbs' life-size portrait of Whistlejacket literally stopped traffic when it was projected onto the side of the Sainsbury Wing of London's National Gallery in 1997.

The rearing stallion could be seen from across Trafalgar Square, as if he was about to leap from the building, nostrils flaring, flanks pulsing and eyes wide with fear.

At the time, the image did not endear the National Gallery to the police, as awe-struck motorists paused to stare at the majestic animal. But eight years later, Stubbs' portrait of Whistlejacket is the National Gallery's most popular selling postcard. It is the image which is at the forefront of people's minds when they think about Stubbs.

Today, Stubbs is considered to be the finest painter of horses in Europe, but it is a status which has been a long time coming, the artist having been classified as a sports painter and derided by the art establishment throughout his life and afterwards.

George Stubbs was born in Liverpool in 1724, the son of a leather worker. A self-taught artist, he moved to York in the 1740s and earned a living as a portrait painter and anatomist, teaching anatomy with the surgeon Charles Atkinson at York Hospital.

Like Leonardo da Vinci, he was fascinated by anatomy, which was fundamental to him understanding every sinew of the horse. He shut himself up in a Lincolnshire farmhouse in 1756 for 18 months and began to study the horse in detail.

"He wanted to learn about horses from the inside, which was an extraordinary project," says Susan Foister, curator of the National Gallery's new exhibition dedicated to Stubbs' work.

"With one young female assistant, he would kill the horses, bleed them and inject their veins with wax. He used a system of pulleys to hoist them up and then he dissected them. It must have been very arduous, very messy and very smelly. It seems incredible that he proceeded to do these extraordinarily beautiful horse drawings."

Not satisfied with creating his images, Stubbs produced a book on the subject entitled The Anatomy of the Horse, published in 1766, and, aged 34, he headed to London.

The drawings became his calling card among the British aristocracy and the first of many influential patrons was the Duke of Richmond, who commissioned him to create three sporting paintings. These include The Duchess of Richmond and Lady George Lennox watching the Duke of Richmond's Racehorses at Exercise at Goodwood.

In their distinct yellow and red livery, every detail is captured, and yet, bizarrely, they do not look natural. Stubbs has three of the horses galloping across the plain, with their legs outstretched and hovering above the ground. Although the movement is not how horses gallop, it is how it must have appeared to the naked eye in the 18th century.

Stubbs went on to gain many wealthy and influential patrons including Viscount Bolingbroke, the Duke of Ancaster, Lord Grosvenor and Viscount Torrington. He painted the magnificent Whistlejacket - a winner of the 2,000 guinea race at Newmarket - in 1762 for the Marquess of Rockingham. The stallion's name, it is believed, stemmed from a Yorkshire drink, a mixture of gin and treacle which would have produced a reddish brown colour seen in Whistlejacket's glossy coat.

As the new, faster thoroughbreds were introduced, the demand for Stubbs' racehorse portraits and stud-farm scenes increased throughout the 18th century. Later in his career, he caught the eye of the Prince of Wales, who commissioned 14 paintings in his lifetime. One of these is the 1793 painting of Laetitia, Lady Lade, who can be seen dressed in a blue silk riding habit, handling her rearing horse with the skill and aplomb of any man.

"Lady Lade was a notorious figure who started her career as a servant in a brothel before she attracted the attention of Sir John Lade, the Prince of Wales' racing manager," says Susan. "She certainly looks like she has the measure of it. She was quite a racy character and the Prince of Wales coined an expression 'to swear like Lady Lade'."

In a bid to escape the suffocating stereotype of being a mere horse painter, Stubbs went on to incorporate lions into his work and they can often be seen ravaging his terrified horses. It was his way of showing that art could deal in the terrible and the beautiful.

"He wanted his pictures to be respected as historical paintings, the kind of paintings which were admired in the 18th century," says Susan. "His independence as a scientist and anatomist gave him the drive and ambition to put the horse to one side."

As Stubbs entered late middle age, his finances began to dry up. He had never married but during his time in the North, he had lived with a woman and had four children, the eldest of whom was George Townley Stubbs (who later became his assistant), suggesting his partner's name was Townley. It has been suggested that she was a Catholic with Jacobite connections, which could be a reason why he never married her. Stubbs died in 1806 at the age of 81.

Although he had made a living during his working life, he was never seen as a great British artist and it took until 40 years ago for his star to begin to rise again. In 1960, a Stubbs oil painting fetched £500 at auction. But by December 2000, a portrait of the dapple grey horse Euston went for more than £2.7m at Sotheby's - which was considerably less than the undisclosed sum the National Gallery paid for Whistlejacket.

His rise to latter day fame was partly due to Paul Mellon, an American millionaire who dedicated his life to the arts and owned some of Stubbs' greatest work. In 1984, the first ever major exhibition of Stubbs was held at the Tate Gallery, organised by Judy Egerton, one of Mellon's close advisors.

Now, for the first time in 20 years, the public can have a chance to enjoy the work of George Stubbs once more in a new exhibition at the National Gallery, with many pieces coming from the homes of the landed gentry who were Stubbs' original patrons. It is an exhibition which explores his love and fascination of the horse; a simple sports painter no more.

* Stubbs and the Horse can be seen in the Sainsbury Wing of the National Gallery, London, until September 25. For more information and to book tickets, contact 0870 906 3891 or log onto www.nationalgallery.org.uk.