The fragile parchment text, bearing royal insignia, was sent in hope to a pope by a deposed monarch begging for her life.
Yet while the letter addressed to Sixtus V failed to save Mary, Queen of Scots, from the executioner’s axe, the document itself did survive buried in the depths of the Vatican’s so-called Secret Archives.
Jealously guarded for centuries, it is now 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 8th 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 Queen of Scots wrote her missive from her prison cell at Fotheringay Castle, in Northamptonshire, just three months before she was executed following her long incarceration.
Written in French on November 23, 1586, the Catholic Mary asks forgiveness for her sins, but also rails against perceived falsehoods perpetrated by her enemies in England and warns the pope of treacherous cardinals.
She claims that the tribunal that condemned her to death was illegitimate and also recounts the suffering she had undergone during nearly 20 years of imprisonment under the rule of her Protestant cousin, Elizabeth I.
Mary had fled to England after an unsuccessful attempt to regain the throne in Scotland. She initially sought protection from Elizabeth, but because of the perceived threat she posed as the focus of Catholic plotters who considered her the rightful ruler of England, she was instead imprisoned before being tried for treason.
Any hopes that the pope would come to her rescue were dashed when Mary was put to death at Fotheringay at 8am on February 8, 1587.
The priceless document is normally kept with tens of thousands of others on 50 miles of shelves in climate-controlled rooms in the Vatican’s Apostolic Palace as well as in a high-security underground bunker.
Archivists have gathered them together for an exhibition to be held in Rome’s Capitoline Museums to celebrate the 400th anniversary of the founding of the Secret Archives in the present form.
Father Federico Lombardi, the Vatican’s official spokesman, said: 'It’s an exceptional event.
Never have so many documents from the Secret Archives been allowed to leave the Vatican.'
Organisers of the exhibition, entitled Lux in Arcana: The Vatican Secret Archives Revealed, said it will “recount history through its sources'.
British visitors to the exhibition may be equally interested by an appeal by the Westminster Parliament, bearing the red wax seals of more than 80 English lords, cardinals and bishops, asking the Pope to annul Henry VIII’s marriage to Catherione of Aragon.
Sent to Pope Clement VII in 1530, it failed to resolve the dispute, which eventually led to religious schism and the founding of the Church of England.It will be displayed alongside documents from the heresy trial of Galileo Galilei, whose scientific theories that the Earth revolved around the Sun attracted the hostility of the Catholic Church in the early 17th century.
One of the most unusual documents is a letter written on birch bark in 1887 by the Ojibwe Indians of Ontario, Canada, to Pope Leo XIII.
Other previously unseen documents relate to Pope Pius XII, who was pope between 1939 and 1958 and who has been accused of not doing enough to speak out against the Holocaust during the Second World War.
The Vatican has until now stubbornly resisted calls from historians and Jewish groups to release papers from Pius XII’s controversial papacy.
All the papers, stamped with seals which read ‘Archivio Segreto Vaticano’, are normally stored in the fortress-like building tucked behind St Peter’s Basilica, its approach lined with Swiss Guards in ceremonial uniform.