The most portentous decision for every new prime minister is what to write in the secret “letter of last resort” to Trident submarine commanders telling them what to do with their nuclear missiles if the British government is wiped out. In Monday’s debate on the renewal of Trident, Theresa May should tell parliament what life-or-death decision she has made in her letters of last resort.

It is said that Margaret Thatcher ordered our nukes, trained on Moscow, to be fired so as to cause maximum destruction to the enemy – ie to its civilians. That order, even for a nuclear “second strike”, would today be illegal.

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It is ironic that although Chilcot produced so much condemnation of Blair for joining an unlawful war, MPs are now being asked to vote for a weapons system that cannot be used without committing a crime against humanity. This was defined in 1998 by the Rome Statute, which set up the international criminal court, as “a systematic attack directed against a civilian population, resulting in extermination or torture, or an inhumane act intentionally causing great suffering”.

The same statute additionally makes it a war crime to intentionally launch an attack in the knowledge that it would cause incidental loss of civilian life or severe damage to the natural environment, out of proportion to military advantage.

Trident’s 200 thermonuclear bombs, each 10 times more powerful than those that struck Hiroshima and Nagasaki, are illegal because they cannot discriminate between military targets and hospitals, churches and schools; because of their capacity to cause untold human suffering for generations to come; and because their consequences (eg ionising radiation, which tortures victims and lingers for half a century) are beyond the control or knowledge of the attacker, who cannot judge the proportionality of their use.

As the international court of justice put it, back in 1996: “The destructive power of nuclear weapons cannot be contained in space or time. They have the potential to destroy all civilisation and the entire ecosystem of the planet.”

So why is our law-abiding government spending tens of billions on a weapons system that cannot lawfully be used?

First, because its advisers wrongly think that nuclear weapons are legal in certain circumstances. Back in that 1996 case, the UK argued that it could lawfully drop “a low-yield nuclear weapon against warships on the high seas or troops in sparsely populated areas”.

This scenario has now been shown up as fantastical: “first use” in these circumstances by the UK would trigger a nuclear reprisal with inevitable damage to the atmosphere, the oceans and the “sparsely populated” area (which would henceforth be entirely unpopulated). In any event, Trident’s weapon-bays will not carry “low-yield” bombs, and if they did the result would be better achieved by conventional weapons, making nuclear deployment unnecessary and disproportionate.

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The world court ruled that the threat or use of nuclear weapons would “generally” be contrary to war law but might be lawful “in extreme circumstances of self-defence, in which the very survival of a state would be at stake”. This was a time-warped view of war law in 1996 that is not tenable today. The court, to be fair, predicted as much, saying that it expected international law to “develop” towards a total ban on the use of the bomb. It soon did, with the Rome Statute and subsequent development of the principle that a state has no right to preserve itself at the expense of damage to other states and to the rights to life of millions of citizens.

It is absurd to suggest that it would have been lawful for Hitler, his back to the bunker wall, to start a nuclear Götterdämmerung to save the Nazi state (Nuremberg decided it was not lawful for him even to fire doodlebugs). Given what we now know about the uncontrollable and devastating propensities of modern nuclear weapons, it is unlawful to fire them at all.

There is a further legal reason for allowing Trident to wear out. It is Article VI of the nuclear proliferation treaty (NPT), by which parties undertake to proceed in good faith to “general and complete” nuclear disarmament.

The world court’s 1996 ruling decided that this imposed not a “mere” obligation but a binding legal obligation on existing nuclear states to reduce the number of their bombs gradually, to zero. It is contrary to the spirit of article VI to upgrade rather than downgrade the fleet.

A decision to phase out Trident would help Britain recover some of the clout it has lost through Brexit. It would show moral leadership, and shame other nuclear powers that have failed to live up to their NPT obligations (especially the US; President Obama’s Nobel prize was prematurely awarded in part for envisaging “a world without nuclear weapons”).

Moral leadership from a nuclear-weapons state is urgently needed. The latest US defence budget allocates $1tn for future modernisation of its nukes and it has acquired new sites for them, in Poland and Romania. President Putin has promised in return a new generation of nuclear-tipped intercontinental ballistic missiles. The American most knowledgeable on the subject – Bill Clinton’s defence secretary William J Perry – has just published a book warning that “nuclear doom” is closer today than it ever was during the cold war.

Although possession of nuclear weapons is not per se unlawful, the UK is under a duty to reduce its arsenal: the vice of refurbishing Trident is that it encourages other states to do the same, and remains a constant stimulus for countries – particularly in the Middle East and Asia – to acquire arsenals of their own.

When negotiating to buy Polaris (Trident’s predecessor), back in 1962, Harold Macmillan confided in his diary that “the whole thing is ridiculous”, but consoled himself with the thought that “countries which have played a great role in history must retain their dignity”.

A half-century later, the best way for Britain to regain its dignity post-Brexit is not to throw vast sums of money away on a weapon that cannot lawfully be used, but rather to appear as the first of the “big five” powers to shoulder its legal obligation to disarm under article VI of the NPT. It will be many years before the mushroom cloud becomes a hallucination, but at least Britain would be able to boast that it had led the way.