The death of Jean Moulin at Klaus Barbie's headquarters in Lyon has haunted the French for more than half a century and was one of the biggest mysteries of Vichy France. He was captured, along with other prominent Resistance members, on June 21, 1943, when a meeting in the southern town of Caluire was raided by the Gestapo. The search for who betrayed one of France's greatest national heroes has continued ever since.

It has now apparently ended with the publication of the "secret last testimony" of Lydie Bastien, the lover of one of Moulin's closest associates, René Hardy. Hardy was an activist with the Combat Resistance group. He was the only Frenchman to escape from Caluire and soon came under suspicion of treachery.

But two trials were abandoned for lack of evidence and Bastien, a devotee of Nietzsche and the occult, fought passionately to clear his name. In doing so, she was also ensuring her own safety. For the true architect of Moulin's downfall, according to her own private confession before her death - only now published in a book by Pierre Péan, the historian - was Bastien herself.

At the age of 20, she infiltrated and betrayed the inner circles of the French Resistance using Hardy's obsessive love for her as a means of entry. Blackmailing the lover she despised, she orchestrated the arrest of Moulin through Hardy and was paid in diamonds and gems stolen from Jews. She wore them until her death in 1994.

The trap which was sprung on Moulin by Bastien in June 1943 was conceived in January the same year, when she first met Hardy, who was an admired Resistance fighter. It was a love affair sponsored by the Germans from the start. "I was already working for the Germans and they told me how to find and get to know Hardy, who I met and seduced at their request," Bastien told a confidant, almost 50 years later.

Hardy was to lead to Moulin, but first he had to fall hopelessly in love. It did not take long. Bastien said: "He was soon obsessed with me. Very quickly he was telling me all his secrets and I had complete access to his secret files." Having fallen in love, Hardy then fell into the hands of the Germans, who on June 7 arrested him on a train to Lyon, for which Bastien had bought the ticket.

"The fool even thought I was coming with him," she remembered. "I warned the Germans he was on his way. Then Barbie called me to Lyon where they had taken Hardy and I told him that he had no choice. He had to collaborate with Barbie or my parents and myself would be arrested as well."

Hardy, cornered and fatally besotted with Bastien, agreed to give information on Resistance activities. In doing so he came under the control of Harry Stengritt, a handsome 31-year-old Gestapo officer with whom Bastien had begun a love affair before she met Hardy. Bastien read Hardy's mail and Stengritt used her information to extract more details from him.

Once the Caluire meeting on June 21 was discovered, Moulin's fate was sealed. Hardy led the Gestapo to the house and staged his own escape. He later shot himself in the arm to pre-empt accusations of treachery. Moulin died after being tortured in Barbie's cells at Lyon.

After the war, Bastien re-invented herself as a Buddhist mystic, living for a while in Bombay and then in the United States where, under the patronage of Eleanor Roosevelt, the wife of the ex-president, she founded a centre for research into "the nature of man". Hardy meanwhile lived unhappily under the cloud of suspicion.

Then, in 1988, Bastien decided to disclose her past to Victor Conté, a Vietnamese of French extraction, to whom she gave permission to pass on the information after her death. After keeping silent for five years, Conté approached Péan secretly last February. For Péan, the author of a previous book on the mysteries of Moulin's death, the 56-year-old riddle was finally solved.

Aware that he was relying on the testimony of one witness, he wrote an afterword to fellow historians explaining his decision to trust Conté's account. Péan said: "Conté's testimony confirmed and deepened my previous investigations into the death of Moulin. I had already identified Lydie Bastien as a key figure in some way. The testimony helped me to understand the crucial role of Harry Stengritt in the Caluire arrests."

It also "confirmed longstanding suspicions and theories" of Resistance figures, who had maintained that she had "framed" Hardy. Conté's description of a photograph of Bastien between two German soldiers - the existence of which was known only to Péan and one witness - was final confirmation of the authenticity of his account.

Approaching his death in 1987, Hardy clearly understood that he had been deceived. Years earlier, in 1971, when snubbed by an old Resistance comrade he shouted angrily: "You had the good fortune never to meet Lydie Bastien! She was a divinity sent from Hell."