The idea that the UK’s entire command and control apparatus, and even the country as a whole, could be wiped off the face of the earth in a single nuclear barrage is chilling. Yet it is this possibility that prompted a dark and sobering tradition among Prime Ministers following the advent of the UK’s “second strike” nuclear deterrent—their nuclear ballistic missile submarines.

As soon as a new Prime Minister takes office, they receive highly classified briefings regarding the country’s nuclear weapons capabilities—specifically on the massive destruction that a single Vanguard class nuclear ballistic missile submarine and its 16 Trident II D5 missiles can convey. They also hear about the immense amounts death the UK’s potential nuclear armed adversaries can deliver on a moment's notice. Afterwards, and in conjunction with their top military advisor, the Prime Minister pens four identical letters in their own hand writing outlining instructions on what to do if a nuclear exchange has occurred and the British government, including the Prime Minister and a designated second person, has been killed or incapacitated. Under the worst of circumstances, the Letter of Last Resort would stand as the last act of Her Majesty’s Government.

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Once the letters are written, they are transferred to the four Vanguard class nuclear ballistic missile submarines that makeup the UK’s nuclear deterrent. Onboard, they are stored inside double nested safes and will remain there until the Prime Minister decides to change the orders or their reign ends. After which time they will be destroyed in sealed condition so that nobody knows what the orders were. During the time following a perceived attack, a series of progressive checks are made by submarine crews leading up to the captain opening the letter. These include trying to listen for radio transmissions from various levels of Royal Navy and Ministry of Defense command using multiple methods, and most famous of all, listening for new radio broadcasts by BBC Radio channel four, and specifically new episodes of BBC Today. If all else fails, following a certain predetermined period of time, rumored to be a matter of a few days, the skipper of the Vanguard class submarine will move toward extracting the letter and executing its blind instructions. Daniel Martyn Commanding Officer of HMS Vigilant next to a safe that contains the Prime Minister's last resort instructions, on board Vanguard class submarine HMS Vigilant (AP photo):

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There are a handful of known instructions the Prime Minister can choose from when penning their Letters of Last Resort, and a combination of them can also be stated. These commands include retaliate, don’t retaliate, use your own judgement, put yourself under the command of the US Navy if it still exists, or go to the same in Australia. Clearly the secrecy surrounding the letters is key as without knowledge of what the fail-safe second-strike commands are, the nuclear deterrent remains credible. If a Prime Minister’s command is to not retaliate, it may be because they have gamed out that if the letter is opened, the nuclear deterrent has already failed. Thus raining down more destruction will only result in more lives lost and lower chances that Earth can recover from the heavy radiation, fallout and nuclear winter that may follow. Obviously under most situations commands to launch would be delivered via an emergency action message, that can even be conveyed via America's VLF communications capabilities. The Prime Minister would have just minutes to order a retaliatory strike through established channels before the enemy’s warheads detonated over London and other population centers, as well as key government and military installations around the UK. In a Daily Mail expose, Peter Hennessy and Richard Knight described the room where this order would be received and pushed out to the Vanguard class submarine on patrol: “Were the order to fire to be given from a still-functioning government, it would come to CTF 345, a bunker known as ‘The Hole’ at the Northwood facility in Middlesex. There, inside a perimeter of intense security, a small group of Naval officers sit waiting for that unlikely communication. We went to CTF 345, the first journalists ever to visit, to meet those men. They are, like their colleagues in Vanguard, quietly efficient, businesslike and matter-of-fact. They explained that the heavily encrypted message arrives on a computer from wherever the Prime Minister had issued it — which would most probably be the Government Emergency Room in a bunker deep beneath the MoD in Whitehall. The officers then collect codes from two safes attached to the far wall of the ops room. Those safes are monitored constantly, via CCTV, by Royal Marines. If anyone approaches them who is not properly authorised to do so, the Marines will storm the room with weapons ready. The officers of CTF 345 showed us how they would, in pairs, authenticate the message and, finally, put it onto the transmission system for communication to the submarine on patrol, together with the encrypted coordinates of the designated target.” Alternatively, if a surprise strike wiped out the UK’s ability to communicate with their Trident missile equipped boats, one of which is on patrol in the Atlantic at all times, the Letter of Last Resort can work as a sort of automated fail-safe response that ensures the second strike capability is activated as a means of vengeance. The trigger that fires D5 Trident SLBMs aboard a Vanguard class SSBN. It is made out of the grip of a Colt 45 (AP photo):

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