Letters of last resort

[2] If the letters are not used during the term of the prime minister who wrote them, they are destroyed unopened after that person leaves office, so that their content remains unknown to anyone except the issuer.

[3] A new prime minister writes a set of letters immediately after taking office and being told by the Chief of the Defence Staff "precisely what damage a Trident missile could cause".

[4] The documents are then delivered to the submarines in sealed envelopes, and the previous prime minister's letters are destroyed without being opened.

[7] 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.

[12] The KGB's attempts to obtain the contents of the letters of last resort are part of the plot of the BBC Cold War spy drama The Game (2014).