Sometimes from the shore you catch glimpses of them. Sinister dark shapes sneaking out of the Gareloch to secretly patrol the oceans, threatening havoc on an unimaginable scale.

The UK government’s four Vanguard submarines, armed with Trident nuclear missile systems, regularly leave their base at Faslane near Helensburgh to perform what the Ministry of Defence calls “continuous at sea deterrence”.

For decades it has not been clear what they are meant to be deterring, or whom they are targeting. Iraq? Russia? North Korea? Al Qaeda? Islamic State? They haven’t stopped foreign invasions, nuclear test explosions, planes flying into towers, suicide bombers in marketplaces, gunmen in concert halls - or a lorry careering along a crowded seafront on the French Riviera.

The justification for threatening to kill millions by flattening and poisoning multiple cities has never seemed so useless, so stupid and so immoral. And that’s before we even consider the mammoth cost, at £167 billion and rising.

When the UK is facing deepening austerity as it exits from the European Union, why is it even contemplating wasting so much money on renewing a weapon that is so supremely pointless? The money could be much better used to alleviate poverty and ill health, improve the country’s ageing infrastructure, and build up our conventional defences.

Even the argument that the UK needs Trident like some kind of phallic symbol to shore up British power and influence in the world is based on a false premise. As we report today, it is more American than British, and crucially dependent on US satellites, software and hardware.

Britain’s continued possession of the ultimate weapon of mass destruction gives other countries an excuse to develop their own. If instead it started to disarm and joined the vast majority of countries in agreeing to ban nuclear weapons in talks planned over the next few years, the world would become a safer place.

Scotland was never asked if it wanted nuclear weapons on its territory. Every recent test of public opinion suggests that the majority of Scots - and of Scottish politicians - do not want it.

Renewing Trident tomorrow, which the UK parliament looks likely to do, will not only prolong a senseless, costly and malign defence policy, it will also deepen the already huge gulf between Holyrood and Westminster.

Renewal will further underscore the need for independence, and bring yet more Scots voters to the Yes cause. In seeking to bolster its nuclear swagger, the UK will be hammering yet another nail into the coffin of a doomed union.