Royal Navy vessels monitored Russian warships as they began to move through the English Channel en route to a suspected bolstering of the bombing campaign against the besieged Syrian city Aleppo.
Defence Secretary Sir Michael Fallon said the deployment of the Russian convoy was aimed at testing British capabilities and that it would be marked "every inch of the way".
The Russian vessels, including the smoke-belching aircraft carrier Admiral Kuznetsov, were shadowed by the Navy as they passed through the Dover Strait - one of the busiest seaways in the world.
A Ministry of Defence spokesman said: "They are all in one line now. They have to be to effectively go through the traffic separation scheme, and they are progressing as expected.
"They haven't slowed or sped up. They are going to be past Dover in the next few hours. When they are clear of the Dover traffic separation scheme, they will probably break back out in a formation scheme and be on their way.
"We still don't know where that is and how they are going to get there, but everything so far has been exactly how we would have expected."
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 moved south.
The Royal Navy's monitoring of the Russian flotilla coincided with Trafalgar Day, the day when Britain triumphed in the Battle of Trafalgar on October 21 1805.
The Russian passage through the Channel came after 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 PM 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.
Asked whether the Russian deployment was an indication that Moscow regarded Britain as weak, a Downing Street spokesman said: "I would reject suggestions that the Russians feel we are too weak. Clearly, we are not weak at all."