The research by Simone Bertière, a specialist in the lives of France's seventeenth and eighteenth-century queens, shatters the myth of a semi-impotent, foppish king, and a sluttish queen, favourite targets of scurrilous pamphlets that inflamed the mobs of 1789. It also undermines the most influential biography of Marie-Antoinette, written by Stefan Zweig in Vienna in 1932 after he discovered uncensored correspondence between the queen and her domineering mother, the Empress Marie-Theresa.

'Since then, the presumed impotence of Louis and his cowardice in refusing an operation to correct a small physical malformation have been accepted as a matter of fact, sufficient to explain the queen's neurotic instability,' Bertière said, commenting on her 700-page biography, Marie-Antoinette, l'insoumise (the rebel). 'But Zweig did not compare these letters with those sent by the Hapsburg ambassador to the empress which leave no doubt at all that Louis XVI did not suffer from malformation.'

It was not until seven years after marrying Louis XV's orphaned grandson, then the Dauphin, at Versailles in 1770 that Marie-Antoinette, 'a little girl paralysed by terror', lost her virginity. From the first fruitless night the physiological realities which, according to Bertière, nineteenth and twentieth-century historians glossed over, were the object of intense court records, letters and diplomatic exchanges that described their sexual characteristics in detail. But despite the opinion of surgeons that the couple dodged conjugal activity because it was too painful for both of them, Louis's mother-in-law empress insisted that the problem lay wholly with her son-in-law's inadequate penis.

'I refuse to believe that it is my daughter's fault,' the empress wrote to her ambassador at Versailles, maintaining demands for an operation on the king even after several intimate inspections by doctors. They repeatedly said there was no evidence of phimosis, a narrowing of the preputial orifice, a theory that Zweig insisted on after reading nagging letters from the imperial mother-in-law replying to misleading correspondence from her daughter.

Concern at the failure to consummate a marriage, essential for a military alliance between the Bourbons and Hapsburgs, was a matter of recorded clinical analysis from the first weeks. By 1772, Louis XV, notorious for his love life and generous genitalia, tackled his grandson, a virgin at marriage, about his barren union. Louis, then 18, told him that he had tried several times to deflower his wife 'but was always stopped by painful sensations'.

A year later, Louis achieved what was called a 'demi-succès', telling his grandfather that Marie-Antoinette was now 'my wife' after a rare night in the same bed. But she was still considered a virgin in 1777 when Austria's Joseph II, the queen's older brother, questioned the couple about their failure to produce an heir. The Austrian ruler then wrote to his brother Leopold to say that the French king, who succeeded to the throne in 1774, 'had well-conditioned, strong erections and introduced his member, stayed there for two minutes without moving, withdrew without ejaculation, and then, still erect, wished [his wife] good evening. He should be whipped like a donkey to make him discharge in anger'.

Bertiere said that for both king and queen, sex was an 'abominable task' for which the only possible explanation was the physical disparity between them. It was only after more strong words from Joseph II that the unhappy pair conceived a child, a daughter born in 1778, the first of four births, including the future Louis XVII who died as a prisoner in the Temple after his parents were executed in 1793.

Bertière said it would be wrong to blame Marie-Antoinette for sparking off the revolution but 'by her flightiness she hastened the monarchy's discredit'.

'Her conjugal failures, abundantly spread in public, added ridicule to the real virtues of Louis XVI while his complacent attitude towards her completed his reputation for weakness,' the author added.