It weighs over 10,000 tons, it is 135 metres long, and it carries half a dozen two-megaton ‘Domesday’ torpedoes capable of triggering a radioactive tsunami large enough to ‘plunge Britain to the depths of the ocean’. No wonder the world held its breath last week as Russia’s latest nuclear-armed submarine departed Severodvinsk Port to begin trials in the White Sea, 140 miles from the Finnish border.

Named after Russian explorer Yerofey Khabarov, the Khabarovsk will now undergo rigorous testing before being considered fit for active service.

The 100-strong crew as well as a team of onshore engineers will stress-test everything from mooring protocols to propulsion, steering and diving capabilities, as well as the integration of sonar and onboard computing systems. The vessel is eventually expected to travel east to join Russia’s Pacific Fleet, which itself launched a series of missile tests last week from the disputed archipelago of islands north of Japan - triggering a major diplomatic incident.

So what would the Khabarovsk role be in a future conflict? Why is Russia spending a rumoured billion dollars on this vessel when its navy already boasts at least 60 active submarines?

And it is not the only one, it is just one of a mooted trio of new nuclear-armed subs that could be unleashed on the West.

While the design of the Khabarovsk is based on Russia’s Borei-A class nuclear ballistic missile submarines that are a cornerstone of Moscow’s nuclear deterrent, it is, in fact, a bespoke ‘special purpose’ vessel in a class of its own.

Khabarovsk is the first submarine designed with a widened bow to accommodate six Poseidon nuclear torpedoes, each measuring 20 metres in length, two metres in diameter and weighing 110 tonnes each. The only other submarine capable of launching these ‘Domesday’ weapons is the RFS Belgorod, which can carry three Poseidon torpedoes, half the number of the Khabarovsk.

Unlike traditional surface-to-air missiles which risk being intercepted in the sky, Poseidon warheads travel underwater at speeds of roughly 70 knots, 1000 metres below the waves. And thanks to an onboard nuclear reactor that propels them, they have an essentially unlimited range. They aren’t just missiles but autonomous, manoeuvrable vessels in their own right.

The idea for Poseidon torpedoes was first conceived by Soviet engineers as far back as the 1950s, though the plans were scrapped as the technology did not exist to fit the weapons with their own propulsion system.

Unlike traditional weapons, the torpedoes are designed to self-detonate before reaching their target. It is not the blast itself that is the danger, but the subsequent tsunami of radioactive water triggered by the two-megaton bombs – more than one hundred times the strength of the Little Boy bomb dropped on Hiroshima.

And while countless thousands would no doubt be drowned in the initial surge or killed by collapsing buildings, the after-effects of the radioactive flooding would render any land within two hundred miles uninhabitable for generations and result in the deaths of millions.

No surprise then that the Kremlin believes Poseidon torpedoes are the future of its nuclear deterrent. Indeed, Dr Richard Connolly, a military expert on Russia and associate fellow at the Royal United Services Institute, describes Poseidon as a ‘boutique capability’ that has been designed only for the ‘very worst of scenarios’.

Most alarming of all, some Russian commentators have even called on the Kremlin to unleash these missiles on Britain in retaliation against the government’s support for Ukraine, notably last week’s decision to share the blueprint for our Storm Shadow cruise missiles.

Disconcertingly, the Russian Embassy in the UK has announced that the greater Britain’s support for Ukraine, ‘the higher the price it will pay’.

Regardless, before Vladimir Putin could launch his Poseidon torpedoes, he needed a submarine capable of doing so. In the summer of 2012 Moscow awarded a billion-dollar contract to the 125-year-old Russian subs maker Rubin Design Bureau, which duly began construction on Khabarovsk in 2014.

The submarine was set to be sea-worthy by 2020, but severe delays meant it wasn’t until November last year that the submarine first emerged from its construction hangar in Severodvinsk, around 600 miles north of St Petersburg.

‘There is nothing like this in the world in terms of the speed and the depth of the movement of this vehicle – and it is unlikely there ever will be,’ Putin declared at the time, before adding ominously that there were ‘no ways to intercept’ it.

It remains unclear why the construction has been so greatly delayed, though the Covid pandemic and economic pressures resulting from Russia’s 2022 invasion of Ukraine are likely to have played a part.

The Khabarovsk itself has a range of around 6,200 miles, a top speed of 32 knots and an operational depth of up to some metres.

The sub requires a crew of 100 and can be at sea for up to 120 days. Crucially, of course, given that Poseidon missiles are autonomous and fitted with a nuclear reactor and sonar system, the Khabarovsk does not need to be anywhere near the target of a proposed strike, making it even more dangerous to Russia’s enemies.

Unlike most propeller-driven submarines, Khabarovsk’s propulsion is derived from what is known as a single-shaft pump jet. The pump moves the sub forwards by first drawing in water, then ejecting it backwards.

Traditional propellers cause cavitation in the water – the forming of small bubbles – which create underwater noise easily detected by enemy subs. The pump jet system therefore makes the Khabarovsk much harder to identify.

When the first images of the submarine were released by Russia, the pump jet had been painstakingly covered with timber and canvas to conceal its engineering.

The sub also has two additional thrusters for low-speed manoeuvres such as docking or navigating treacherous waters.

As for the six Poseidon launchers, they are positioned at the bow of the ship, which has been enlarged from those of previous attack vessels and splits the warheads across two separate flooded hangers on the port and starboard sides. The hangers are fitted longitudinally, meaning the weapons will be fired out the front of the sub. Conventional, non-nuclear torpedoes occupy the space between the two Poseidon hangers.

And yet, it is clear that this is not the end of Russia’s submarine ambitions. Intelligence agencies understand that two further nuclear-armed subs are already under construction. In 2017 work began on the Ulyanovsk while a third vessel, Orenburg, is understood to have been ordered.

More alarming still is quite how ill-prepared Britain is to deal with this latest threat.

‘During the Cold War we were all over the Soviet submarine programme like a rash,’ Admiral Lord West told the Daily Mail this week. ‘But we’ve not been spending enough on defence, and now Putin has taken advantage of that.’

‘It is certainly worrying that the Kremlin is willing to spend a great deal of money to modernise and improve weapons systems,’ the former First Sea Lord and Chief of the Naval Staff continued, ‘especially when Russia is struggling financially. It rather makes Putin look like a Bond villain.’

Except, of course, that this Bond villain operates on the world stage rather than the silver screen. Khabarovsk and its arsenal of six Poseidon torpedoes may be a weapon of last resort, but how long before the Kremlin’s war machine feels it has nowhere else to turn?