A Premier League player

George Stubbs has long been recognised for his peerless portraits of animals. But, argues Paul Bonav

The National Gallery ushers in its summer season this year with an exhibition that celebrates the work of George Stubbs, one of the most under-valued painters in the history of western art. Previous shows on Stubbs have included work across all the subjects he addressed. "Stubbs and the Horse" is the first to focus on the theme that has come to be seen as the truest mark of his ability.

Bringing together 34 of his horse paintings, as well as examples of the anatomical drawings that established his reputation in artistic and equestrian circles, "Stubbs and the Horse" concentrates on the works for which the artist is best known, and explains the social and cultural environment in which he operated. In the scholarly catalogue of the exhibition, the curator Malcolm Warner discusses how horses were regarded in Stubbs's age, Stubbs's classicism, his horse-and-lion compositions and the creation of the thoroughbred in 18th-century England; and Robin Blake examines the young Whig noblemen who were Stubbs's first patrons, as well as the grooms, jockeys, trainers and other attendants who appear in his horse portraits, and his dealings with the Prince of Wales. The book also includes an illuminating essay by the conservators Lance Mayer and Gay Myers on Stubbs's experiments with wax and enamel.

What is it about Stubbs that makes him so good? Certainly nobody has bettered him in depicting a likeness of the horse. Stubbs harnessed the materiality of paint in order to arrive at seamless, naturalistic images of his equine subjects. As Warner confirms, he "never strove for painterly flair - he delighted in the exquisite descriptive touch". Stubbs isn't a great painter despite his descriptive interest in the thoroughbred, but precisely because of it. What is often overlooked is his ability to render the human figure and capture the essence of landscape with a mastery that has few peers. As a consequence, he is persistently excluded from the canon of leading international artists.

A contributory reason for this debarring, and this is something that Stubbs shares with William Hogarth and William Blake, is linked to his very Englishness. A strong identification with one country has always proved an impediment to an artist's global reputation. Stubbs's pictures of racehorses and hunters served the interests of the English landowning nobility. In return for their custom, he rewarded his patrons with equine portraits of uncommon authority. Stubbs won the acclaim of the nobility, as the catalogue makes plain, because he gave it what it needed: a clear-sighted vision of the England that surrounded it, a world that it had fashioned to its own satisfaction.

Stubbs's fascination with nature and nature's relationship with mankind is reflected in his obsessive attention to detail, and nowhere is this more evident than in his remarkable book, The Anatomy of the Horse. His career as a painter of horses was rooted in his knowledge of equine anatomy. In his early thirties, he spent 18 months dissecting and drawing the bodies of horses at a remote farmhouse in Lincolnshire. Out of this gory, Enlightenment-driven industry came the sublime Anatomy, and a steadfast commitment to the pursuit of reality.

The Anatomy of the Horse features 18 plates etched by the artist from his drawings, and more than 50,000 words of meticulous scientific text. Its publication in 1766 earned Stubbs instant and lasting appreciation, not least from the animal paint-ers who followed him. "Try to imagine," wrote the 20th-century equestrian painter Sir Alfred Munnings, "Stubbs at his work setting up and dissecting horse carcasses in the barn . . . making detailed drawings for plate after plate with all the names of the muscles, and finally engraving each plate himself, this latter part of the work an entirely new departure for him, being spread over something like a period of six years. We may then begin to grasp the magnitude of this labour of love."

Notwithstanding Stubbs's belief in scientific inquiry as the basis for art, his Anatomy-derived portraits on Newmarket Heath of the racehorses Eclipse, Gimcrack and Turf are not just paintings of record. His canvases and works on paper are so much more than exact descriptions of material possessions because they engage the personality and feed the spirit. They compel examination. To see Stubbs's work solely as a reflection of the aspirations of his aristocratic clients is to neglect the phenomenal impact of his aesthetic achievement.

The centrepiece of the current show is the National Gallery's own Whistlejacket. With this portrait of a volatile, rearing stallion, alongside his scenes of placid mares and foals on plain grounds, Stubbs generated an abstraction in painting every bit as radical as Picasso's cubism and Mondrian's neoplasticism.

Numerous commentators have referred to the emblematic quality of Whistlejacket, and it can be read as a cipher. The horse certainly elicits comparison with his equine predecessors from the world of medieval heraldry. Further- more, the painter's studies of mares and foals, deployed in frieze-like parades of profile archetypes, evoke close associations with precedents from classical antiquity and ancient Egyptian tomb painting.

Whistlejacket was commissioned by the 2nd Marquess of Rockingham, one of Stubbs's regular clients, who terminated work on the picture before it was completed with the addition of a rider and landscape. It could be argued that Rockingham was the catalyst behind the artist's sensational innovation of isolating objects from their context, yet he would never have arrested the painting's development if Stubbs had not presented him with a picture that represented an unparalleled leap forward in the depiction of natural form.

If Whistlejacket is a work of genius, and most critics think it is, it is obliged to share that accolade with Stubbs's other life-sized composition of a horse: Hambletonian, Rubbing Down, his study of the great racehorse recovering after winning at Newmarket in 1799. I can think of no other canvas, in all of British art, that matches Stubbs's late masterpiece in its pictorial, intellectual and emotional accomplishments. It is a supreme painting, and its relative obscurity is a conundrum.

I understand that Hambletonian, Rubbing Down was unable to travel from its home at Mount Stewart, in County Down, for conservation reasons. This is a pity, because if it had, visitors to the show would have had a unique opportunity of judging it against John Constable's The Hay Wain and J M W Turner's The Fighting Temeraire, those time-honoured giants of English art on permanent display at Trafalgar Square.

Painted when Stubbs was 75 years old, Hambletonian, Rubbing Down signifies the climax of the artist's concern for the figure, and his late-flowering interest in comparative anatomy. The huge mass of post-race horseflesh threatens to overwhelm the trainer and groom who also appear in the painting only to find themselves relegated to its margins. Hambletonian is restrained by his human companions, but his heaving flanks and troubled demeanour overshadow the landscape and everything in it - bodies and buildings, earth and sky.

Even if Stubbs had never produced another picture as dazzling as Hambletonian, Rubbing Down, this magnificent painting would have assured him a prominent position in western art. As it is, he handed down to posterity some of the most profoundly affecting pictures ever coaxed from the tip of a brush, and he stands shoulder to shoulder with Hogarth and Blake, Constable and Turner as one of the world's finest painters.

Paul Bonaventura is senior research fellow in fine art studies at the University of Oxford

"Stubbs and the Horse" will be at the National Gallery, Trafalgar Square, London WC2 (020 7747 2885) from 29 June to 25 September. The exhibition is organised by the Kimbell Art Museum, Fort Worth in association with the Walters Art Museum, Baltimore and the National Gallery, London