The Royal Navy has confronted a Russian craft as it cruised towards the English Channel, and is now escorting it away.
The submarine was detected by a combined effort of NATO allies as the Navy remains vigilant for Russian incursions into the seas around Britain.
The incident comes as NATO commenced Operation Anakonda in Poland to reassure eastern European nations following Russia's annexation of Crimea in 2014.
The Russian sub is a Kilo-class vessel named Stary Oskol.
A Russian Kilo-class submarine
The class comprises diesel-electric attack submarines, produced in Russia, for anti-shipping and submarine operations in shallow waters.
Co-ordinated effort by NATO members may have been required to track the sub because of the anechoic tiles of rubber and synthetic polymer that distort sonar sent out to detect them.
These subs are usually armed with surface-to-air missiles to attack aircraft, torpedoes or mines to attack ships and other subs, and Club-S cruise missiles.
Cruise missiles can deliver warheads to targets on land, but the Club-S variant is designed as an anti-ship/anti-sub weapon.
Royal Navy vessel
Escorting the sub is HMS Kent, a Type 23 Frigate, which had been out to sea already as part of the commemoration for the Battle of Jutland.
These are versatile craft with a range of weaponry, including general electronic jamming and decoy systems, as well as naval and machine guns.
For their anti-ship role, Type 23s have Harpoon anti-ship missiles, Mark 8 naval guns, and space on deck for a Westland Lynx helicopter with additional anti-ship weapons.
HMS Kent crashing against the waves
Surveillance radar and Sea Wolf missiles are part of the anti-air armament, while they use Sting Ray torpedoes for anti-submarine warfare.
Westland Lynx helicopters can also be equipped with Sting Rays.
A Sting Ray torpedo being fired from Type 23 frigate HMS Westminster during an exercise
This is the latest peak in tensions between NATO and Russian militaries – Russian jet fighters have also flown close to US warships recently, and reports of British airspace and waters being violated by Russia are becoming increasingly common.
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