Moscow has deployed its top secret super-sub the 430ft Kazan, which is capable of carrying 40 Kalibr cruise missiles.
The Kazan, Russia's most advanced ballistic submarine, entered service in 2018 and underwent extensive trials last year before finally being deployed a month ago.
It sailed from the Northern Fleet naval base at Murmansk, and is currently in the North Atlantic, escorted by up to five Akula-class submarines.
To counter the threat, the Navy has deployed its latest Astute-class subs, backed up by Trafalgar hunter-killer boats. Sources say they have been tasked to find and shadow the Kremlin's vessels. "In effect, both Russia and Nato are putting each other to the test," said a senior Royal Navy intelligence source.
"The increased number of spy ships is a classic tactic indicating Russian subs are in the area.The usual configuration is to deploy two decoys with the Kazan to make sure she is not found but we understand as many as five have deployed on what appears to be a major surveillance operation.
"From Nato's point of view, the objective is to locate the Kazan and visibly monitor it - to show there is no chink in our maritime armour."
Two nuclear-powered Trafalgar hunter-killers and two Astutes slipped out of Faslane naval base, in south-west Scotland, two weeks ago. They are now working with at least one US Los Angeles-class attack submarine.
NAVY CHIEFS are involved in a Cold War-style "cat and mouse" chase with Russian submarines in the North Atlantic, sources have confirmed.
www.express.co.uk