The letters of last resort are the letters written by the UK Prime Minister to the naval officers in command of the four nuclear submarines which provide the UK's nuclear deterrent, containing instructions in case of the destruction of the government & the incapacitation of the PM. They are written as soon as possible after there is a change of Prime Minister, and the previous letters are said to be destroyed without being opened.
What is known about the process behind the creation of the letters? Has any Prime Minister or insider ever revealed their contents? Does the PM have to pick from a list of options, as this Guardian article implies, or can the PM write whatever he/she sees fit?