Spare a thought for Daniel Mytens. In 1625 he was appointed “picture drawer ... to Charles I”. By the end of the decade, he was leading artist to the court of the King of England. Not bad for a first-generation immigrant, the son of a Dutch coach-builder. But in 1632 the wheels abruptly came off his carriage when a painter called Anthony Van Dyck, protege of Pieter Paul Rubens, came to London from Flanders. The contest of rivals soon came to a head. Mytens painted a competent if rather dull allegorical portrait of Charles I receiving a laurel wreath from his queen, Henrietta Maria. The King commissioned a second version of the same subject by Van Dyck and was so pleased with the result that he appointed him “principalle Paynter in Ordinary to their Majesties”. Van Dyck was knighted and given a house on the river at Blackfriars. Mytens, his reputation eclipsed, slipped quietly back to Holland and into the shadows of history

“Van Dyck in England”, a manageably medium-sized exhibition at Tate Britain, starts with work by such lesser lights of early seventeenth-century English art. Robert Peake’s Portrait of Princess Elizabeth, daughter of James I and Anne of Denmark, is a characteristically ornate example of the brittle late Mannerist style that prevailed in elite portraiture at the turn of the century. The solemn-faced ten-year-old girl, trapped in her lace ruff and minutely decorated silver-thread dress, looks as stiff as a playing-card queen. Mytens’ own portrait of Charles I, painted in 1628, is at least more convincingly human. The King wears an elaborate costume of maroon trimmed with gold ribbon and stands on a chessboard floor painted in textbook perspective. It is a sombre, sober, almost impressive picture, although its mannequin monumentality was soon be exposed by the incomparably superior art of Van Dyck.

Tate Britain’s show cunningly reenacts the revelation, swiftly calling a halt to the pot-pourri of rivals and predecessors. The second of the show’s galleries is devoted to Van Dyck’s own royal portraiture, a subject which might itself furnish enough material for an entire exhibition. ‘The Great Peece’, of 1632, one of the largest pictures Van Dyck ever painted, dominates one long wall. The king and his consort sit in a pair of red plush chairs in a portico open to the sky, forming a family group with their two young children. Charles I is sober enough, by his standards, in dark velvet and salmon-coloured silks. Henrietta Maria wears an informal dress of radiant yellow, like a ready-to-wear sunrise. Studied informality is carried off to perfection. Two dogs behave perfectly while the children are like angels. King and queen govern their household as easily and naturally as they govern the larger family that is the nation. Van Dyck has contrived the composition so that the King is aligned perfectly with a towering classical column above him. Behind him, seen at a distance along a stretch of twilit Thames, lie Parliament House and Westminster Hall. He is the pillar of the state, the master of all that he surveys. Or at least he thinks he is.

Another great set-piece portrait on loan from the Royal Collection is Charles I on Horseback with M. De St Antoine, of 1633. As stormclouds part behind him to reveal patches of bright blue sky, the King rides through a triumphal arch on a milk-white thoroughbred. His French riding-master, teacher now become pupil, gazes up at him with an adoring gaze of the sort usually reserved for saints contemplating visions of Jesus Christ. But Van Dyck’s pictures of Charles I are, in a sense, religious paintings. According to the theory of the divine right of kings, Charles I was God’s representative on earth. In the words drilled into him by his father, James I, he was “a little GOD to sit on his throne and rule over other men”. Such themes were constantly re-enacted in Stuart court masques and ceremonials, allegories of anarchy averted and chaos replaced by order. It was the ease with which Van Dyck translated such political metaphysics into images of effortless grace that enthralled his royal patron.

Part of the pathos of Van Dyck’s royal portraits stems from the knowledge that these are images of pride before its fall. With hindsight it is tempting to see weakness as well as fatal inflexibility in the gaze of the King. The growing insecurities of Charles I’s reign are certainly reflected in the changing patterns of Van Dyck’s royal propaganda. In many of the later portraits, Charles is depicted in armour, symbol of military power but also the sign of his increasingly defensive, beleaguered state of mind. In the famous portrait of Charles II as Prince of Wales, even the fresh-faced boy prince is encased in a suit of burnished steel.  

Van Dyck in England was far more than painter to Charles I. Enormously prolific, he eventually became painter by appointment to the whole Stuart court. He immortalised the swagger of Lord John Stuart and His Brother, Lord Bernard Stuart, twinned epitomes of the long-haired cavalier beau, dressed in peacock finery, strutting through the world with arms akimbo and elbows out-thrust (a gesture of natural superiority, according to a fashionable continental manual of deportment, because it implied the act of “pushing one’s way through crowds”). He perfectly caught the deadly ruthlessness of Thomas Wentworth, blackly serious in a black suit of armour as he pats the head of a meek Irish wolfhound – symbol of the country he had not so gently subdued on behalf of the English crown. He painted Thomas Killigrew, the picture of melancholy, accompanied by an infinitely sympathetic, anonymous friend.

Van Dyck gave an unprecedented sense of intimacy to the British portrait. From Rubens, who taught him, and from Titian, whom he loved, he also brought to the genre a new and unfamiliar sense of drama. He was the first painter of English faces to create portraits that make the viewer wonder what will happen next. He was also a technical magician, using virtuoso flourishes of impossible drapery as the pretext for abstract colour compositions that have fascinated artists from Thomas Gainsborough, to John Singer Sergeant, to Howard Hodgkin today (the exhibition includes a truncated tribute to his influence on later generations). Last but not least, he was one of the first artists to paint the direct, unashamed sex appeal of a woman flirting. The nonpareil example of this is probably his portrait of Lucy Percy, Countess of Carlisle, of 1637. The lady might have been recently bereaved, but still she raises an eyebrow to invite the viewer behind the arras. She looks as though she might have rather enjoyed the sittings.

Rumour has it that Van Dyck’s jealous mistress, the splendidly named Margaret Lemon, shooed many a dove-eyed milady out of his studio. At Tate Britain, Van Dyck’s portrait of her is hung side by side with that of his wife – an arch invitation to compare and contrast, or at the very least discuss. Lady Van Dyck, in a dress of purest Virgin Mary blue, fingers her rosary beads while appearing to dart the painter a sharp look. Mistress Lemon, alluringly clad in flimsy classical fancy dress and accompanied by a winged Cupid, suggestively caresses the bulbous helmet of a suit of armour. It is a mischievous juxtaposition but one which might have amused Van Dyck himself. He too had feet of clay. No one is perfect. It should be every portrait painter’s motto.