Russia has tested a giant new nuclear submarine in open waters for the first time, just days after a tense standoff in the Black Sea with Britain involving a Royal Navy destroyer.

The ‘Belgorod’, believed to be the largest submarine developed anywhere in the world in 30 years, was trialled in the White Sea over the weekend, according to Russian state media.

Once approved for use, the vessel will be capable of launching nuclear strikes with six intercontinental ‘Poseidon’ torpedoes.

It will also act as a mothership for smaller, deep-diving submarines with robotic arms that can tamper with or even cut vital cables that lie on the seafloor.

The Royal Navy announced earlier this year that it would deploy a ‘spy ship’ specifically to stop such submarines sabotaging Britain’s internet through such tactics.

The Belgorod, the exact specifications of which are not known, is currently undergoing manufacturer’s tests and is set to be handed over to the Russian state towards the end of 2021.

It is set to serve in the country’s Pacific Fleet, sources close to the Russian Defense Ministry told state media.

But there are concerns in the West that it will be deployed to the Arctic and North Atlantic, where Moscow has significantly increased its submarine activity in recent years.

Last month, Defence Secretary Ben Wallace told the Telegraph that Moscow was the UK’s “number one adversary threat” and that Britain’s waters were “regularly visited” by Russian ships.

Russian naval assets have been detected by the UK more than 150 times since 2013.

The testing of the Belgorod comes just days after a confrontation with Britain in the Black Sea, when a Royal Navy defender passed through contested waters.

It was an area that Russia has claimed as its own since its annexation of Crimea in 2014. Britain still recognises it as Ukrainian territory.

Russia said it fired warning shots and dropped bombs close to the ship, a claim disputed by Downing Street.

General Sir Nick Carter, the chief of the defence staff, later warned that any misstep in such a standoff could lead to all-out war, describing the risk of an “unwarranted escalation” as “the thing that keeps me awake at night”.

Russia also said at the weekend it was monitoring the American missile destroyer USS Ross, which entered the Black Sea on Saturday.