Russia has deployed 10 heavily armed submarines on a mission skirting UK territorial waters and heading for America.
The flotilla, including subs bristling with nuclear weapons, mines, ballistic missiles and torpedoes, was signed off by President Vladimir Putin.
Eight of the 10 are nuclear-powered. They left secretive bases on the Arctic Kola Peninsula a few days ago, passing close to north Scotland.
It is the largest underwater operation launched by a Russian leader since the end of the Cold War.
Norway’s intelligence service is trying to track its progress with increased surveillance flights.
Norwegian sources believe Russia’s aim is for the fleet to penetrate as far as possible into the North Atlantic without detection by NATO, sailing submerged north of Scotland through the Greenland-Iceland-UK gap.
The flotilla raises challenges for NATO and threatens its maritime re-enforcement and re-supply lines across the Atlantic.
It comes as Russia’s most advanced new nuclear-powered submarine test-launched its first Bulava intercontinental ballistic missile, hitting a target thousands of miles away.
The launch was conducted in the White Sea off Russia’s north west, with a dummy payload reaching a test site in the far eastern Russian region of Kamchatka.
The Borei-class vessel, part of a different fleet, was submerged when it fired but the escalation in aggression comes amid arms-control tensions with the West.
It was seen as a bid to show how Russia is able to defend its own bases while threatening the US Eastern Seaboard.
The MoD said: “The UK takes appropriate measures and action to ensure the integrity of our waters and to protect our nation’s interests. The Royal Navy is always ready to safeguard our waters.”
Expert Bruce Jones said the flotilla was a “significant defence ‘challenge’ to the UK as well as the US and NATO”.
He added: “It shows up how numbers of our warships have been drastically reduced and our warning systems across the North Atlantic have become degraded.
"This is a very serious development indeed.”
