Hilary Spurling unravels the tale of Ther se Humbert, who lied her way to fortune and almost destroyed the French Third Republic in the process

I first came across Ther se Humbert by chance six years ago, in the course of research in and around Toulouse for a biography of Henri Matisse. She was the heroine of the Humbert affair, described by a future prime minister of France in the 1890s as "the greatest swindle of the century".

La Grande Ther se, whose outrageous lies (and even more outrageous hats) knocked Paris sideways, turned out to be one of the great con queens of all time. She was a cross between Madonna and a female Mr Toad, with touches of Robert Maxwell thrown in: flamboyant, manipulative, impossible to resist and so dangerous that the full extent of her operations, hushed up at the time, has been virtually written out of French history.

Her story astounded me, not least because at the end she seemed to vanish into thin air. After a sensational downfall, a public flight across Europe pursued by the press of two continents, eventual arrest and a trial in 1903, Humbert was sentenced to five years solitary confinement with hard labour. Nothing more was heard of her. At the time, the Humbert affair was felt to threaten the stability of an administration still reeling from the impact of the Dreyfus affair. Today Humbert is remembered dimly, if at all, as a grotesquely comic figure.

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When I first stumbled across Humbert in 1993, the locals of her native Languedoc seemed at best vague about her. That first encounter was odd enough to be worth a trip to the departmental reference library in Toulouse to look her up in turn-of-the-century newspaper reports. A friend who was with me settled down with the the city's radical newspaper La Depeche and I read Le Telegramme. We turned the pages with mounting incredulity.

Ther se was the daughter-in-law of Senator Gustave Humbert, one of the founding fathers of the Third Republic, a passionate defender of democracy and the secular state, drafter of the constitution and keeper of the seals. His impeccable credentials lent weight to her claim to have been lent a fortune of 100,000,000 francs by American millionaire, Robert Henry Crawford (rumoured to be her natural father). The only snag was that the millionaire's two litigious nephews disputed Madame Humbert's claim in lawsuits that rose slowly up through every court in France. All three Crawfords (and of course their wealth) proved to be figments of Humbert's imagination.

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But for two decades before that, bankers, businessmen and celebrities from the Empress Eugenie downwards fell over themselves to lend her money. "If she had laid claim to an inheritance of no more than four or six million, she would not have lasted two years and would with difficulty have managed to raise a miserable few thousand francs," wrote an admiring contemporary. "But a hundred million! People took their hats off to a sum like that I and their admiration prevented them from seeing straight."

Paris in the closing decades of the 19th century saw an explosion of energy and confidence comparable to what would happen in America in the 1920s. Industrialists who built railroads, mechanised production and pioneered the first department stores were used to rich returns on rash investment. Madame Humbert flourished at the heart of this new world of lavish expenditure and prodigal display. She owned a chteau with its own hunting grounds in the forest of Fontainebleau and a palatial mansion on the Avenue de la Grande Armee, an extension of the Champs-Elysees in Paris.

These were the actual equivalents of the dream castles she had built as a child as the daughter of a penniless, disgraced and bankrupt father in Toulouse. She remained a fabulist all her life. People were bewitched by the power of her imagination. Successful lawyers and hard-headed company directors found themselves stuck fast in the web of fantasy she spun around the strongbox said to hold the Crawfords' non-existent bearer bonds. If she had chosen fiction, instead of real life, as the medium for her romantic stories about mysterious deeds, locked coffers and surprise legacies, she would have been a 19th-century bestseller.

As it was, she lived out her youthful daydreams of a world richer, grander and more highly charged than anything reality could offer. Her hats caused sensation at the opera. Her costumes were stuck with jewels. Romantic interest came from her elegant younger sister Maria, always a hot topic of Parisian gossip as the putative fiancee of one or other of the phantom Crawfords.

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The indiscriminate richness of Humbert's taste was crucial in persuading investors to subscribe to her illusion. She embodied not only her own but other people's dreams. Her retinue included three successive presidents of the republic and half a dozen prime ministers as well as senior judges and top civil servants. The attorney general, the chief of the Paris police force and the head of the Paris Bar were her intimate cronies.

Her father-in-law's name supplied great prestige. It also meant she had the judiciary in her pocket. Creditors could be silenced, lawyers bought off, awkward customers arrested and sent down. For years, the road to republican privilege and power ran through the Humberts' house. People who protested generally ended up ruined, bankrupt, in some cases dead. But eventually even Humbert's colossal confidence buckled.

A press campaign was mounted in Le Matin, panic spread among investors, a Parisian magistrate ordered the opening of the famous strongbox. Ten thousand people gathered on the Avenue de la Grande Armee while locksmiths broke into the safe, which turned out to contain nothing but an old newspaper, an Italian coin and a button.

People reacted at the time with the same disbelief that I felt when I first stumbled across this story in Toulouse. Its rise and fall have the simplicity, clarity and logic of the best fairytales. Contemporary journalists went to town on its larger-than-life aspects. Parisians whistled Humbert pop songs, played Humbert board games and blew up Humbert-shaped balloons.

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Subsequent brief outlines of the Humbert scam have ignored the exuberant inventiveness that provoked an answering collusion in Humbert's audience, or at least some degree of collaboration in a collective dream. Stripped of its imaginative dimension, her career looks equally dismal and squalid. No one has tried to disentangle what actually happened. There has been little attempt to explore the political implications and none to assess the involvement of Senator Humbert.

Nor has anyone ever noticed the Humberts' connection with the artist Henri Matisse, who married their housekeeper's daughter in 1898 and celebrated his wedding in the house on the Avenue de la Grande Armee. Matisse's career was dislocated and his private life laid to waste by the Humberts' exposure, or rather by its effect on his wife's family. When I came to write the first volume of the Matisse biography I had to fight so hard to relegate Ther se to a minor role that, once it was done, I felt I owed her a book of her own.

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Le Grande Therese: The Greatest Swindle of the Century (Profile) is published this week, Pounds 7.99.