Their legendary love pact - they never married but swore mutual devotion to each other with the freedom to have affairs - was an attempt to overthrow the stifling hypocrisy that, for so long, had dictated most people's lives.

Yet a fascinating new book paints this supposedly high-minded duo as serial seducers bent on their own gratification and as a couple who used their apparently lofty philosophy as a springboard to excuse their multiple liaisons, often with under-age teenagers who were broken by the experience.

And while Simone de Beauvoir preached her ideal of feminist independence and equality, eschewing such 'bourgeois' concepts as marriage and children, and claiming women should behave just like men, the truth is such a lifestyle made her bitterly unhappy and she became obsessively jealous over Sartre's countless conquests.

In this sordid relationship of supposed equals, he was always one step ahead of her - though it didn't start that way.