Philip de Who? Sadly for someone who saw himself in an illustrious line of foreign-born artists including Holbein, Van Dyck, Lely and Kneller, and who specialised in portraying British royals and nobles, De László's name has slipped well below the horizon since his death in 1937. Style recognition, however, remains high in his society portraits, with their bravura brushwork: sweetly glamorised, chiffon-draped images of Princess Marina and the late Queen Mother when Duchess of York; European kings, dukes and generals in full regalia, vestiges of vanished hierarchies. And thousands of them, for De László perfected the art of instant painting alla prima on to canvas, to capture the likeness in a single sitting, like a flattering cartoonist with gestural panache.

His male portraits put an acceptable face on plutocracy. Where Sargent, his real role model, lamented that each work cost him a friend, De László gained friends on all sides. "The portrait of my wife has a ray of heaven illuminating in her face the charming qualities of her soul," the Duke of Portland wrote. He made his sitters "look exactly as they would like themselves to look", observed the magazine Apollo. No wonder he was popular.

Who was he? A man of several incarnations, born Fülöp Laub in Budapest in 1869, the son of a poor tailor. He claimed to have left school at nine to work successively for theatrical scene-painters, an architectural sculptor and a porcelain-painter before colouring up prints in a society photographer's studio  a portent of the future, perhaps. Evening study led to fine art training, then Munich and Paris. His scenes of peasant life were comparable to those being produced in Brittany and Newlyn and, following the national trend, he hungarianised his name to László at about the same time as he swapped his Jewish faith for Catholicism. The exact date at which he did so is unknown, but he certainly enjoyed pork and sauerkraut in Munich.

And portraits brought home the bacon, with a commission from Prince and Princess Ferdinand of Bulgaria. Socially, he felt he was "walking on eggs", but swiftly became a charming courtier, dressed as a dashing Magyar with boots, cape and great moustaches. Pictures of Ruritanian barons and bankers' daughters led to portraits of the German chancellor, Prince Chlodwig zu Hohenlohe-Schillingsfürst, and Pope Leo XIII. Soon, Felipe László, as he appeared in the magazine the Studio in 1900, built a grandiose house in Pest and married Anglo-Irish Lucy Guinness (of the banking, not the brewing branch), whom he had met in Munich.

"The brain, the hands, the conversational powers, the looks  God has given more to him than ordinary mortals," she gushed in 1909, by which time they were settled in London and he had been reinvented as Philip Alexius de László. Their descendants, through an eponymous foundation, have recently set out to revive his reputation, with exhibitions, catalogue raisonné and this biography. Largely uncritical, it nonetheless quotes Lucy's private complaints of ill temper and imperious fault-finding, which began soon after their marriage. But she endured, as a loyal wife and the mother of five sons.

Setting his sights on royalty, he targeted the rich and titled, often through their womenfolk. Not all his work was successful: the portrait of Vita Sackville-West quite misses her singularity. But that of Elinor Glyn, with cobalt eyebrows to match her jewels and cascading satin, brilliantly conveys the vulgar glamour of the Edwardian age. "Can't you see the powder on her face?" bitched its first viewers. "And the hair, plainly dyed! He is so clever."

However, pride comes before a fall. The reader can hardly forbear to cheer as the chronicle of fashionable commissions and connections culled from De László's incoming letters and press cuttings is interrupted: in 1917, he was arrested as a hostile and potentially dangerous citizen. Having taken British nationality, he had unwisely continued to send money to his family in Hungary, then inside enemy territory.

Three weeks in prison and six months' internment in an old workhouse followed, along with social disgrace: he was expelled from Hankley Common Golf Club, and Eton refused to admit his sons, despite his influential friends. One wants to know more of the leading artists who welcomed his downfall; was it envious schadenfreude or something more sinister?

Without this episode, the story would be a dull read. But beneath the surface, other narratives are glimpsed. That of a talented man driven to pursue wealth from Vienna to New York in the very period when crowned heads and their financial peers were being toppled by war, revolution and market collapse. And that of a painter who realised too late he had chosen a cage, not a platform. In 1920 he aspired to the grand manner  Europa mourning her fallen sons  only to find that formulaic portraiture had driven out his technical and conceptual skills. Though his facility came close to being facile, one should not be too snobbish about his work. The fluent, spontaneous air of his work still catches the eye in a country house hallway. A small display this summer at the National Portrait Gallery showcases his stylishness, including a pretty view of the Queen Mother with "marble shoulder" underlining her blue blood. Will his fame rise again towards that of his admired predecessors, Reynolds, Gainsborough and Thomas Lawrence, as this book so optimistically hopes?

The Legend of Elizabeth Siddal by Jan Marsh is published by Quartet.