What would you do in the event of nuclear war? It’s one of the first questions that new prime ministers face – and one of the most chilling. Tony Blair is said to have been “very quiet” when asked, and few of the country’s top brass are willing to reveal their answer.

Jeremy Corbyn has been in the headlines again this morning for his vocal opposition to Trident, the UK’s nuclear deterrent system at sea. The fact he would not use nuclear weapons is unlikely to come as a shock – he is Vice Chair on the Campaign for Nuclear Disarmament’s 2014-15 council – but it is still remarkable to hear such a forthright response on a matter most leaders have declined to comment on.

Yet it's a decision all prime ministers must make. The incoming premier is usually tasked with writing the four “letters of last resort” within a few days of taking office. These handwritten notes are taken to the UK’s four Vanguard-class submarines, the ships which carry the ballistic missiles the Royal Navy calls “the nation’s ultimate weapon”. At any time, one of these submarines, letter on board, is patrolling somewhere in the world’s oceans. Since 1969, the Navy has not missed a single day of patrol.

The letters contain the PM’s instructions of what to do in the worst-case nuclear scenario: the obliteration of the UK state. If they, and their selected second in command, are both killed in a nuclear strike, the commander of the Vanguard submarine undertakes a series of checks (one of which, pleasingly, is whether Radio 4 is still broadcasting – a mechanism which reportedly once put the Navy on alert when the station went off air for fifteen minutes). If all the checks come up negative, the note dictates what the Captain should do. As a 2009 Slate article on the subject remarked, it's a strangely intimate way of handling the country’s nuclear capability – akin to life imitating John le Carré – and Britain is the only nuclear power to use such a system.

But what have former prime ministers written? Slate reports that unused letters are destroyed without being read: the weightiness of the decision, perhaps, being offset by the fact that no-one will ever know what one chose except in the most desperate circumstances. The inevitable time delay between the letter being written and its being opened means that the PM must excise broad moral judgements rather than situationally-dependent ones; this is about ethics, not tactics. Given both the secrecy of the letters and the bathos of any reveal – what is of the utmost gravitas in the event of nuclear war can read as bizarrely quaint in peace-time – few have elected to comment on their selection.

Most of what we know about former PMs’ judgements comes from a Radio 4 program, The Human Button. This documentary suggests that the four options available include not only retaliation and non-retaliation, but putting Trident in the service of an allied government – Australia or America – and asking the captain to excise his own judgement. Denis Healey, who was one of Harold Wilson’s alternate decision-makers, said he “had to make you think you would use them, even if you wouldn’t in practice. . . . In practice I would have not”. Only one prime minister has said his decision on the record: Jim Callaghan.

If it were to become necessary or vital, it would have meant the deterrent had failed, because the value of the nuclear weapon is frankly only as a deterrent. But if we had got to that point, where it was, I felt, necessary to do it, then I would have done it. I’ve had terrible doubts, of course, about this. I say to you, if I had lived after having pressed that button, I could never, ever have forgiven myself.

Various articles online claim that several former prime ministers – including Gordon Brown, Tony Blair and John Major – have said otherwise, arguing that if a nuclear deterrent had already failed then retaliation against civilians would be pointless. But the original source of these reports is hard to ascertain. As it stands, Jeremy Corbyn might be one of the handful of candidates for prime minister whose ultimate decision would be known to the public.

What do you think?