Lord Monteagle, who received the warning letter about the Gunpowder Plot, was named William Parker.
William Parker was the fourth Baron Monteagle, a Catholic peer who nevertheless occupied a place within the political establishment. On 26 October 1605, a mysterious letter warned him not to attend the opening of Parliament because a terrible blow was being prepared there.
Monteagle passed the letter to Robert Cecil, the king’s principal minister. The warning eventually reached King James I, who ordered a search of the cellars beneath the House of Lords. That search found Guy Fawkes with gunpowder during the early hours of 5 November.
The letter’s author has never been established with absolute certainty. Francis Tresham, one of the conspirators, is often suspected, partly because he was Monteagle’s brother-in-law, but the historical evidence does not produce a universally accepted answer. The letter’s warning was therefore crucial even though its authorship remains disputed.