Letters of Last Resort

The letters of last resort are four identically-worded handwritten letters written by the Prime Minister of the United Kingdom to the commanding officers of the four British ballistic missile submarines. They contain orders on what action to take in the event that an enemy nuclear strike has destroyed the British state and has killed or incapacitated the Prime Minister and the "second person" (normally a high-ranking member of the Cabinet) whom the Prime Minister has designated to make a decision on how to act in the event of the Prime Minister's death. In the event that the orders were to be carried out, the action taken could be the last act of the British state.

The letters are stored inside two safes in the control room of each submarine. The letters are destroyed unopened after a Prime Minister leaves office, so what action would have been taken is only ever known to the outgoing Prime Minister.