The identical handwritten letters of last resort are located on each of the four submarines (Picture: Foreign and Commonwealth Office).
One of Boris Johnson's first tasks once he officially becomes prime minister will be to write a letter of last resort, which is carried on board each of Britain's nuclear submarines.
For more than five decades, the UK has maintained a continuous at-sea deterrence - with at least one nuclear ballistic missile submarine on patrol since 1969.
Four Vanguard-class submarines currently carry Britain's Trident missiles and warheads, with the decision to launch them sent through to the boat through an encrypted message in the missile control centre.
The identical handwritten letters of last resort are located on each of the four submarines inside a Russian-doll like safe system and are addressed to the commanding officer of each boat.
This is one of the first things a newly-appointed prime minister is tasked with and offers instructions on what the crew should do in the event Britain is destroyed.
"The Prime Minister is alone when they write the letter," Commodore Bob Anstey, assistant chief of staff submarines, has previously said.
"They get briefed on what it is for, why it exists and then they are left alone, given some paper, a shredder and some envelopes.
"Any drafts they have made, they shred them themselves so no one ever sees it. Then the envelopes are couriered up and put in the safe.
"The previous one from the previous prime minister is shredded without opening, so nobody sees it."
When an order to launch is given, it will usually have come from the Prime Minister and is then turned into an encrypted 15-letter message and sent to the boat.
Once it is received, after being transmitted across all sorts of airwaves to ensure delivery, there is a process on board to decode the message of letters and numbers, and then to verify it.
This is done using crypto books which are kept in the safe system that is located in the control room of each submarine - with part of the message instructing which page to use.
If the order to launch is legitimate, then the commanding officer would proceed to move the submarine into the required position.
A number of keys are then inserted into specific panels on the submarine, with all having to be turned and "lined up" for the system and trigger to work.
Cdre Anstey said the trigger itself is based on the handle of a pistol, is red, and stored in the missile control centre.