A RUSSIAN submarine was spotted off Scotland's west coast before arriving in Cuba last week.
The Ministry of Defence briefed the Prime Minister when Kazan - which did not enter UK waters - was identified by an RAF Poseidon P8 anti-submarine aircraft as it passed Ireland.
The Royal Navy said it routinely monitors waters around the UK to "deter malign activity".
Kazan arrived in Havana along with a number of Russian vessels for a military drill in the Caribbean over the weekend.
HM Naval Base Clyde at Faslane is home to the UK's nuclear submarines as well as other Royal Navy vessels.
A Royal Navy spokesperson said: “The Royal Navy routinely monitors UK territorial waters and the adjacent sea areas to ensure compliance with maritime law, to deter malign activity and to protect our national interests.”
US officials played down the presence of the vessels some 90 miles from Florida, saying they were no threat.
A US intelligence officer told CBS that Kazan was not carrying nuclear weapons.
Cuba's Foreign Ministry identified the other Russian ships as the frigate Admiral Gorshkov, the fleet oil tanker Pashin and the salvage tug Nikolai Chiker.
Russia's defence ministry said Admiral Gorshkov and Kazan are both carriers of advanced weapons, including hypersonic missiles Zircon. They earlier conducted missile drills in the Atlantic.
Russia has sent warships to Cuba in the past and the two nations are long-standing allies.
