JEREMY Corbyn said he wouldn’t press the nuclear button yet the British nuclear deterrent is structured in such a way that a Prime Minister may find himself powerless and his instructions ignored. Trident commanders, the chief of defence staff, and even the Queen are able to stand in his way. So whether Prime Minister Corbyn chose to indulge in murder, mercy or mental instability, the matter might be out of his hands – such is the maddening logic of Britain’s nuclear weapons.

How are the weapons launched?

When a new PM takes office one of his first duties is the nuclear briefing where he meets with the chief of defence staff and is told the hideous truths of how Trident operates and of precisely what it is capable. Away from the camera flashes and cheers, this is probably the moment when his new responsibilities are made hideously clear.

In the 1970s, when nuclear bombers gave way to missiles, the rules on how Britain launched its weapons had to change. No longer was there the luxury of meetings with the war cabinet to discuss retaliation because Russian missiles could reach Britain in minutes – hence the infamous four-minute warning – and so decisions would have to be made swiftly and by the PM alone whether he was in Downing Street, touring a factory, or at a jumble sale in a church hall.

The PM’s decision to retaliate would be conveyed to the Royal Navy bunker at Northwood who would encode it and communicate it to the Trident submarine, one of which is constantly on patrol. The encrypted message would rattle out on a printer in the submarine and this little sheet of paper, instructing Armageddon, would be carried by a sailor who runs with it to the control room. He must carry it aloft, with another sailor running behind him, to ensure he cannot tamper with it.

The commander then decodes the message and gives the order: “Missiles for strategic launch” and soon, with a solemn “click”, the trigger is pressed.

LETTERS OF LAST RESORT

This smooth operation only works if the PM is alive to give the order.

Given that enemy missiles could reach Britain in minutes then it’s easy to assume the PM might be killed. In this case, he’s instructed to authorise a second person to issue orders in his place, but if they are also gone, along with all functioning remnants of the British state, then the PM must give his orders from beyond the grave, via the Letters of Last Resort.

Each new PM is told by the Cabinet Secretary that he must write four identical handwritten letters, one to be placed on each Trident submarine, giving his instructions should Britain face nuclear war.

The contents are unknown to anyone but the PM, and the finished letters are couriered to the submarines where they’re placed inside a safe, which lies inside yet another safe, where they will hopefully never be read.

On the succession of a new PM they are destroyed, unopened and new letters penned.

But when might they be read? Trident submarines can listen to radio communications but may not send any of their own lest they reveal their position. If all radio communication from Britain suddenly falls silent, the Commander must go through a series of checks to determine if his country still exists, as he cannot send a direct message to enquire.

Strangely, one of these checks is the absence from the airwaves of Radio 4’s Today programme. If he is certain Britain has been annihilated then he will open the safes and read the PM’s Letter of Last Resort which apparently gives one of four options: retaliate; do not retaliate; use your own judgment, or place yourself under the command of the American or Australian navy if they still exist.

CAN A PRIME MINISTER’S ORDERS BE BLOCKED?

The strange logic of nuclear deterrence means that what the PM writes in his letter hardly matters; the trick lies in what you imply, not what you intend. If the letter is being read then the game’s already over; deterrence has failed and Britain is a smoking ruin and the handwritten letter becomes simply a quaint, human conceit, an ink-smudged missive from a world already dead.

At this dreadful point, the decision lies with the submarine commander. It will be a time of frenzied horror and unimaginable stress.

Are we to accept that the Commander, knowing his family and home are ashes, will still be standing neat and trim, carrying out his orders unquestioningly, and obeying a state which no longer exists?

Will that little piece of paper actually matter?

But before we get to that awful stage, the PM’s instructions can still be blocked.

If, for example, he’s deemed to be mentally unstable and trying to launch a foolhardy attack then the military can defy him because while a PM may authorise the use of nuclear weapons only the military may order it.

Even the Queen can frustrate a rogue PM, having the power to dismiss him or dissolve Parliament.

What have prime ministers said on the matter?

Tony Blair is said to have “went white” at the prospect of writing his Letters of Last Resort, whereas John Major cleared his diary and retreated to his constituency to write his in solitude.

No PM has spoken publicly about their wishes except James Callaghan who insisted he would have launched a nuclear attack if it was necessary but would never have been able to forgive himself.

On the contrary, Denis Healy – never PM but perhaps Callaghan’s nominated alternative – said he would not have authorised a launch but certainly had to give the impression that he would.

So, in the mad world of nuclear deterrence, it’s acceptable to be thoughtful and human as long as no-one knows it.