The incident between the British and Russian military follows several incursions by aircraft in skies around the UK.
A Russian submarine – capable of carrying cruise missiles – has been intercepted by a Royal Navy warship in the North Sea.
The vessel was detected by NATO forces and is now being escorted by HMS Kent.
The Ministry of Defence said the Type 23 Duke class frigate will continue to shadow the Kilo-class submarine.
Defence Secretary Michael Fallon said: “This shows that the Navy is maintaining a vigilant watch in international and territorial waters to keep Britain safe and protect us from potential threats.”
HMS Kent’s Commanding Officer, Commander Daniel Thomas, said: “Locating this submarine was a combined effort with NATO allies and shadowing such units is routine activity for the Royal Navy.
“We continue to escort the submarine as it conducts its passage, providing a visible presence.”
The incident is the latest face-off between the British and Russian military following several incursions by aircraft in skies around the UK.