The Royal Navy intercepted a Russian submarine as it sailed towards the English Channel.
The submarine was escorted by the frigate HMS Kent on Tuesday evening and was due to pass the Strait of Dover on Wednesday morning.
It is understood that the Stary Oskol, a Kilo-class submarine capable of carrying cruise missiles and torpedoes, was first spotted in the North Sea, where NATO forces are monitoring the waters.
The MoD said it would continue to be followed by the Duke Type 23-class frigate, which took part in the Battle of Jutland centenary commemorations.
Defense Secretary Michael Fallon said: ‘It shows that the Navy maintains vigilant surveillance in international and territorial waters to keep Britain safe and protect us from potential threats.’
The Commanding Officer of HMS Kent, Cdr Daniel Thomas, said: “The location of this submarine has been a combined effort with NATO allies and monitoring such units is a routine activity for the Royal Navy.
“We continue to escort the submarine as it makes its passage, providing a visible presence.”
The incident is the latest standoff between the British and Russian military after several plane incursions into the skies of the UK.
In April 2015, a trawler was dragged violently by its nets while fishing 18 miles off the coast of Northern Ireland, raising suspicions it had snagged a Russian submarine .
However, the Royal Navy later said a British submarine was responsible for snagging the trawler’s nets.