Amongst the first tasks undertaken by a new Prime Minister of the United Kingdom is to write four identical letters, known as the letters of last resort. These secret messages contain instructions on how the UK’s nuclear deterrent, housed aboard four Vanguard-class nuclear submarines, is to respond in the case that the United Kingdom falls or the Government is destroyed by a nuclear strike. If carried out, it is likely that they would represent the last official act of the Government of the United Kingdom.
The United Kingdom is amongst just eight sovereign nations confirmed to possess nuclear weapons (alongside the US, Russia, France, China, India, Pakistan and North Korea). The UK’s current nuclear weapons are carried by four nuclear-powered ballistic missile submarines of the Royal Navy, collectively known as the Trident Nuclear Deterrent. Built specifically for the task of carrying nuclear warheads at Barrow-in-Furness shipyards between 1986 and 1999, the four submarines are HMS Vanguard, HMS Victorious, HMS Vigilant and HMS Vengeance. Each carries up to 16 Trident II nuclear ballistic missiles. While all four are based at HM Naval Base Clyde on the west coast of Scotland, at any specific time at least one will be away on patrol. Their nuclear engines means their operational range is only limited by food and maintenance requirements, allowing them to patrol, undetected, across the globe.
After taking office, a new Prime Minister writes four identical, hand-written letters to be delivered to the commanding officers of each submarine. Before writing them, the PM is instructed by the Chief of Defence Staff just what they are committing to when ordering the use of nuclear weapons, and what damage a Trident missile is capable of. They are then left alone with just paper, pen and envelopes, along with a shredder to destroy any drafts. The letters remain unopened aboard the vessels, housed in secure safes. When a new prime minister is elected and fresh letters are written the previous ones are destroyed unopened, leaving their contents a secret. While the specific messages are unknown, a 2008 BBC Radio 4 documentary stated that there are four known possibilities. These are:
- Retaliate with nuclear weapons against the aggressor
- Do not retaliate
- Use the commander’s own judgement
- Place the submarine under the command of a designated allied nation. Both the United States and Australia were mentioned in the documentary as potential designated allies
The letters are only to be opened and their instructions carried out in the event that both the prime minister and their designated second in command (usually a senior member of the cabinet) have been killed or incapacitated by an attack on the UK Government. While the process used by the submarines to establish if the Government has fallen are not officially stated, one method is said to be for them to establish whether BBC Radio 4 continues to broadcast. Procedure for the older Polaris-class nuclear deterrent that was superseded by Trident was more clear, with commanders told to open the letters if their is evidence of a nuclear attack against the United Kingdom or if no naval communications are received for four hours.