The fragile parchment, bearing royal insignia, was sent to a pope by a deposed monarch begging for her life.

While the letter addressed to Sixtus V failed to save Mary, Queen of Scots from the executioner’s sword, the document itself has survived buried in the bowels of the Vatican’s secret archives for more than 400 years.

It is among 100 of the most historically significant items of confidential correspondence due to go on public display for the first time in a special exhibition in Rome.

The priceless collection spans more than a millennium, from the eighth century to modern times, and features a cast of historical characters who have crossed swords with a succession of pontiffs, from Knights Templar to Galileo, Martin Luther and Henry VIII.

Mary wrote the missive from her prison cell at Fotheringhay Castle in Northamptonshire just three months before she was executed following her long incarceration.

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