A French author's English alter ego has been unmasked by new letters, according to Oxford University.

Professor Nicholas Cronk, director of the university's Voltaire Foundation, said two of the 14 letters shed light on the extent of Voltaire's interactions with the English aristocracy.

In one letter the author even signs his name Francis Voltaire - something which has never before been recorded, he said.

Professor Cronk, who is also a lecturer in the medieval and modern languages faculty, said the letters have been made available online in the Bodleian Library's electronic enlightenment project.

He said: "Voltaire spent two important but relatively undocumented years in England in his early thirties at a time when he was best known as a poet - he arrived with only a recommendation from the British Ambassador to Paris.

"While here, he was exposed to ideas of English writers and later took empiricism back to the continent where it became the basis for the enlightenment.

"These newly-discovered letters are therefore very interesting because they show how Voltaire's close interaction with the English aristocracy exposed him to enlightenment ideas and help us to piece together the nature of those interactions."

Professor Cronk came across the letters while carrying out research in the New York Public Library, the University of Morgan library and the library at the University of Columbia.

François-Marie Arouet, better known by his pen name Voltaire, was an influential thinker and writer and a strong advocate for social reform in the late 18th Century.