The Drive noted that upon taking office, every new prime minister is provided a highly classified briefing about current threats and risks to the country's national security. As part of that briefing, the report said, the new PM is told about the massive amounts of destruction the Vanguard sub fleet can inflict on an enemy, as well as the amount of death and damage that can be inflicted on the UK itself.
"Afterwards, and in conjunction with their top military advisor, the Prime Minister pens four identical letters in their own handwriting 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," The Drive reported. "Under the worst of circumstances, the Letter of Last Resort would stand as the last act of Her Majesty’s Government.
"Once the letters are written, they are transferred to the four Vanguard class nuclear ballistic missile submarines that make up the UK’s nuclear deterrent," the report continued. "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."
In the moments after a perceived nuclear attack, several progressive checks are carried out by sub crews that precede the moment the captain opens the letter. These checks include attempts to hear radio transmissions from various Royal Navy and Ministry of Defense command centers using several methods, with the "most famous of all" being "listening for new radio broadcasts by BBC Radio channel four, and specifically new episodes of BBC Today," the report said. If that fails, after a predetermined amount of time that is believed to be a couple of days, the sub captain will move to extract the letter from its safe and execute the orders verbatim.
The report explained further:
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 judgment, 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.
Of course, under most situations, launch commands would be given via an emergency action message that can be sent via the United States' VLF communications capabilities. In such a scenario, the PM would only have a few minutes to order a retaliation strike through established channels before enemy warheads destroyed London and other UK population centers along with key government and military installations.
In an expose for the Daily Mail, Peter Hennessy and Richard Knight described the room where the order would be received and then pushed out to the Vanguard subs 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.
"Letters of Last Resort are not deployed aboard America’s nuclear ballistic missile submarines, but the US, with its large landmass, extensive command and control infrastructure, and nuclear triad is more survivable to a sudden nuclear attack than the UK," the report noted, adding that some see the Letter of Last Resort as a flawed system.