Letter of last resort

The letters of last resort are four identically-worded handwritten letters from 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 government and has killed or otherwise incapacitated both 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 official act of Her Majesty's Government.

The letters are stored inside two nested safes in the control room of each submarine. The letters are destroyed unopened after a Prime Minister leaves office, so their content remains known only to the Prime Minister who issued them.

The letters are written as soon as a new Prime Minister takes office, after being told by the Chief of the Defence Staff "precisely what damage a Trident missile could cause". These are then delivered to the submarines, with the previous Prime Minister's letters being destroyed without being opened.

In the event of the death of both the Prime Minister and the designated alternative decision-maker as a result of a nuclear strike, the commander(s) of the nuclear submarine(s) (at any time at least one is on patrol) will use a series of checks to ascertain whether the letters of last resort will have to be opened.

According to Peter Hennessy's book The Secret State: Whitehall and the Cold War, 1945 to 1970, the process by which a Vanguard-class submarine commander would determine if the British government continues to function includes, amongst other checks, establishing whether BBC Radio 4 continues broadcasting.

In 1983, the procedure for Polaris submarines was to open the envelopes if there was an evident nuclear attack, or if all UK naval broadcasts had ceased for four hours.

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