A team of codebreakers have cracked secret coded messages in letters written by Mary, Queen of Scots while she was imprisoned by her cousin Queen Elizabeth I, in what experts have said is the most significant discovery about Mary for more than a century.

Fifty-seven letters dating from 1578 to 1584, a few years before Mary Stuart was beheaded on 8 February 1587, have been deciphered. Fifty of the letters revealed scripts that were not previously known to historians.

George Lasry, a computer scientist and cryptographer, Norbert Biermann, a pianist and music professor, and Satoshi Tomokiyo, who is a physicist and patents expert, discovered that Mary, Queen of Scots had written the letters after solving her cipher system using computer and manual techniques.

They found them in the French National Library, whose catalogue had listed the letters as Italian texts from the first half of the 16th century, which experts said is likely why they were not discovered for so long.

Lasry, lead author of the study and a member of the Decrypt Project, aimed at deciphering and transcribing historical codes, described the discovery as “truly exciting”.

“Upon deciphering the letters, I was very, very puzzled and it kind of felt surreal. We have broken secret codes from kings and queens previously and they’re very interesting but with Mary, Queen of Scots it was remarkable as we had so many unpublished letters deciphered and because she is so famous.”

Mary was held in captivity for 19 years before her death. The team spotted several mentions of captivity or “ma liberte” – my liberty, and the name “Walsingham”, thought to refer to Sir Francis Walsingham, Queen Elizabeth I’s spymaster. The letters reveal Mary’s distrust of Walsingham and Sir Robert Dudley, Earl of Leicester, who was a favourite of Elizabeth’s.

In the letters, many of which were sent to the French ambassador to England, Michel de Castelnau de Mauvissiere, Mary complained about her poor health, the conditions of her captivity, and spoke about her efforts to negotiate with Elizabeth for her release. The letters document her attempts to win over some of Elizabeth’s officials with presents.

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Mary also expresses her distress about her son, referred to as “mon fils” or my son, later King James VI of Scotland and James I of England, who was taken away from her at the age of one.

Dr John Guy, a fellow in history The University of Cambridge who wrote the 2004 biography of Mary, Queen of Scots, said the findings are a “literary and historical sensation” and mark the most important new find on Mary Stuart, Queen of Scots for more than 100 years.

“These new documents, amounting to some 50,000 words, show Mary to have been a shrewd and attentive analyst of international affairs. They will occupy historians of Britain and Europe and students of the French language and early modern ciphering techniques for many years to come,” he said.

The authors suggest that other coded letters written by Mary Stuart may still be missing.

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