Royal Navy vessels were monitoring Russian warships as they prepared to pass through the English Channel en route to a suspected bolstering of the bombing campaign on the besieged Syrian city Aleppo.
Defence Secretary Sir Michael Fallon has said the Russian fleet will be marked "every inch of the way" as he claimed the deployment was aimed at testing British capabilities.
The Russian vessels, including the aircraft carrier Admiral Kuznetsov, were being shadowed by the Navy as it headed towards the eastern Mediterranean via the Dover Strait.
Type 45 destroyer HMS Duncan sailed from Portsmouth on Tuesday to "man-mark" the Kuznetsov group, and Type 23 frigate HMS Richmond escorted the group from the Norwegian Sea as it steamed south.
The Royal Navy's monitoring of the Russian flotilla coincides with Trafalgar Day, the day when Britain triumphed in the Battle of Trafalgar on October 21 1805.
The Russian deployment comes as Theresa May condemned Vladimir Putin's aggression in Syria, accusing Moscow of being behind "sickening atrocities" in support of Bashar Assad's regime.
Syrian forces, backed by Russian air power, have agreed a temporary humanitarian truce in Aleppo, but Mrs May urged European leaders to take a firm line against Moscow.
Arriving at her first Brussels summit as Prime Minister on Thursday, Mrs May called for a "robust and united European stance in the face of Russian aggression".
Nato said the prospect of Russia's only aircraft carrier heading to the region does not "inspire confidence" that Moscow is seeking a political solution to the Syrian crisis.
Russian defence minister Sergei Shoigu said in September that the Admiral Kuznetsov-led Northern Fleet would be joining a taskforce in the Mediterranean.
According to the Russian news agency Tass, he told a defence board meeting that the plan was to bolster the Mediterranean fleet's "combat capabilities".
A statement from the fleet to the agency on October 15 said the group also consisted of the Pyotr Velikiy battlecruiser, the Severomorsk anti-submarine ship, the Vice-Admiral Kulakov destroyer and other support vessels.