The British Royal Navy intercepted a Russian submarine as it was crossing the English Channel on June 8 and escorted it through.
The Stary Oskol, a submarine capable of carrying cruise missiles and torpedoes, was first detected in the North Sea by NATO forces. The British Ministry of Defense said it will be shadowed by the HMS Kent until it completes passage.
"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," Defense Secretary Michael Fallon said.
The incident is the latest confrontation between the British and Russian militaries following several incursions by Russian aircraft in skies around the United Kingdom.
The Russian Embassy in London objected to all the fuss over a submarine that Moscow claims was moving above water level at a slow speed.
"Her Majestry's government is making much noise about a routine passage...in international waters," it tweeted. "There is enough space for everyone to navigate there."
The Russian Defense Ministry said the sub is sailing to its home base in the Black Sea fleet after trials with the Baltic Sea Fleet.
Based on reporting by The Telegraph.com, Interfax, and TASS