A long lost series of erotic letters could become a literary hit after a book deal was struck following their discovery in a Paris apartment.

The cache of messages dating from the 1920s, from a young woman called Mademoiselle Simone to her married lover named Charles, are said to become increasingly explicit as their relationship develops.

They were discovered in an apartment by French diplomat Jean Yves Berthault and are now to be published around the world, with deals struck just two months before the cinematic debut of best-selling erotic novel 50 Shades Of Grey.

The book - to be called The Passion of Mlle S - will be published in the UK after publisher Selina Walker acquired the letters for Heinemann.

Berthault, who has edited the letters, is believed to have found them while he helped a friend clear the apartment's cellar.

In an echo of 50 Shades, the letters are said to feature Simone pushing her sexual boundaries to keep her lover satisfied.

He said: "We have no way of knowing who Simone or Charles were, or what became of them, only that their affair ended in heartbreak.

"What we have left is Simone's voice that echoes down the years, her letters, and her sexual relationship with the man she clearly loved. This is a time capsule of a book, a truly extraordinary testament to a period of time and a relationship that was as physical as it was passionate.

"And the fact that it was such a deeply buried secret for all these years makes it particularly special. So to be part of the team that brings it to light and to a wide audience is both exciting and a true privilege."