The Letter of Last Resort is a handwritten letter from the Prime Minister to the commanding officer of each of Trident's submarines.
It contains instructions on what action the vessel's commander should take in the event that Britain is obliterated by nuclear attack and all those in authority deceased.
As far as we know the letter can only say one of four things: retaliate, don't retaliate, for the submarine commander to use their own judgement, or for the vessel to put itself under US or Australian command (if possible). Each new British prime minister must write the letter upon taking office.
The process by which a Trident submarine commander would determine if the British government continues to function includes, amongst other checks, establishing whether BBC Radio 4 continues broadcasting.
Submarines on patrol were reported to have briefly gone on nuclear alert in 2004 when Radio 4 went off the air for 15 minutes due to a powercut.
Lord Guthrie, former Chief of the Defence Staff, recalls briefing the newly-elected Tony Blair on Britain’s nuclear capability when he first entered Downing Street in 1997.
"I think quite honestly, like most prime ministers, he hadn't given a huge amount of thought to what this really meant. And it is actually an awesome responsibility. It really comes home to you that he could, if the circumstances demanded it, create devastation on a huge scale."
How did Blair react? "Well", says Guthrie, "he went quite quiet".
In an interview with the BBC, ex-Prime Ministers Gordon Brown, Tony Blair and John Major stated that their instructions all were that in no circumstances should nuclear weapons be deployed against civilian targets - on the basis that to do so after an attack would be a futile act of vengeance that would wreak unacceptable levels of harm on a civilian population.
In addition, any government that would launch such an attack on the UK would most likely be a dictatorship and it would be immoral to make their people suffer for the acts of an unaccountable leadership.
Jim Callaghan, the former Labour Prime Minister was the only other former leader to share their decision on what order they would have given:
"If we had got to that point where I felt it was necessary to do it, then I would have done it."