First unveiled by Vladimir Putin in 2018, both US and Kremlin officials believe that the torpedo is a new breed of retaliatory weapon.
The 24 metre long torpedo was designed to travel up to 80mph (128km/h) underwater carrying a two megaton nuclear warhead - more than 100 times more powerful than the atomic bomb dropped on Hiroshima.
Christopher Ford, then US assistant secretary of state for international security and non-proliferation, warned in 2020 that the weapons were being designed to “inundate US coastal cities with radioactive tsunamis”.
The US Naval Institute, a military think tank, said the development of Poseidon turned assumptions about submarine-launched nuclear weapons upside down.
The weapon is expected to enter into service around 2027, but has reportedly been blighted by technical difficulties.
In November last year, US intelligence officials reported that initial sea tests had failed.
The torpedo had been due to be tested in the Arctic on the Belgorod - which, at 178 metres long, is the largest submarine in the world.
But the vessel returned to port at Severomorsk, the Russian Northern Fleet base, without any evidence the Poseidon had been tested, according to reports.
Tass reported that the submarine’s crew had since completed tests with models of the torpedo.
Analysts, however, say the Kremlin has a history of exaggerating the capability of newly developed weapons.
Justin Crump, of Sibylline, an intelligence and geopolitical risk company, said: “While the Poseidon is certainly a new form of weapon, it is principally a deterrent in nature, designed to avoid missile defence mechanisms.
“Kremlin-sponsored rhetoric about the weapon has often been overblown, particularly the claims aimed at the UK.”
There are very few confirmed public details of the weapon apart from apparent Russian simulations of it striking enemy aircraft carriers and coastlines.
Last year, a Russian broadcaster played an animated video which purported to demonstrate how a Poseidon torpedo could “plunge Britain into depths of the sea”.
On his Sunday evening prime time show, Dmitry Kiselyov, a leading Kremlin propagandist, said the weapon could be used to turn Britain into a wasteland by drowning it in a 500 metre tsunami of radioactive seawater.
And in response to Britain’s decision to send 14 Challenger II tanks to Ukraine, retired general Andrey Gurulev, another Putin loyalist, said the country should be “demolished from the face of the Earth”.