Letters of last resort
The United Kingdom relies on four Vanguard-class submarines to provide its nuclear deterrent. The submarine can carry 16 Trident II SLBMs, a ballistic missile armed with nuclear warheads with a range of 7000 miles (11265.41KM). The submarine does not have to refuel as she runs on a nuclear reactor and could theoretically remain underwater, circling the globe for approximately 20 years. Only the need for supplies prevents this. At least one submarine is always active and armed, lurking beneath the world’s oceans whilst the other three will either be in re-fit or in port.
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, and would have the capability of destroying human civilization.
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.
In the event of the death of the Prime Minister and the designated alternate decision-maker as a result of a nuclear strike, the commander(s) of the nuclear submarine(s) (of which there is always one 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 Trident submarine would determine if the British government continues to function includes, amongst other checks, establishing whether BBC Radio 4 continues broadcasting.
According to the December 2008 BBC Radio 4 documentary The Human Button, there were four known options given to the Prime Minister to include in the letters:
Retaliate with nuclear weapons;
Don’t retaliate with nuclear weapons;
The submarine commander uses his own judgement; or
The submarine commander places himself under United States or Australian command, if possible.