The Casket letters were eight letters and some sonnets said to have been written by Mary, Queen of Scots, to the Earl of Bothwell, between January and April 1567. They were produced as evidence against Queen Mary by the Scottish lords who opposed her rule. In particular, the text of the letters was taken to imply that Queen Mary colluded with Bothwell in the murder of her husband, Lord Darnley. Mary's contemporary supporters, including Adam Blackwood dismissed them as complete forgeries or letters written by the Queen's servant Mary Beaton. The authenticity of the letters, now known only by copies, continues to be debated. Some historians argue that they were forgeries concocted in order to discredit Queen Mary, and ensure that Elizabeth I supported the kingship of the infant James VI of Scotland, rather than his mother. The historian John Hungerford Pollen, in 1901, by comparing two genuine letters drafted by Mary, presented a subtle argument that the various surviving copies and translations of the casket letters could not be used as evidence of their original authorship by Mary.
The Queen's husband, Lord Darnley, was killed in mysterious circumstances at the Kirk o'Field in Edinburgh on 10 February 1567, and she married the Earl of Bothwell on 15 May 1567. Bothwell was widely thought to be the main suspect for Darnley's murder. The Earl of Moray, Mary's half-brother, and the 'Confederate Lords' rebelled against Queen Mary and raised an army in Edinburgh.
Mary surrendered at the Battle of Carberry Hill on 15 June 1567, was imprisoned at Lochleven Castle, and on 24 July 1567 abdicated. Her infant son was crowned as James VI of Scotland on 29 July 1567 and Moray was made Regent of Scotland. At this time rumours spread that Mary had abdicated because of the discovery of letters which incriminated her. At the end of July 1567, the Earl of Moray, who was in London, told Guzman de Silva, Spanish ambassador to England, that he had heard of the finding of a letter in Mary's own handwriting to Bothwell which implicated her in the murder of Lord Darnley. He had not revealed this to Queen Elizabeth. By the end of August 1567, Edmund Grindal, Bishop of London, had heard that letters in Mary's handwriting urging Bothwell to hurry up with the killing of Darnley had been found in a box of Bothwell's papers, and the Bishop sent this news to the Reformer Henry Bullinger in Geneva.