Russian submarines are hunting down British Vanguard boats in a return to Cold War tactics not seen for 25 years, Navy chiefs have warned.
A specially upgraded Russian Akula class submarine has been caught trying to record the acoustic signature made by the Vanguard submarines that carry Trident nuclear missiles, according to senior Navy officers.
British submariners have also reported that they are experiencing the highest number of "contacts" with Russian submarines since 1987.
If the Russians are able to obtain a recording of the unique noise of the boat's propellers it would have serious implications for Britain's nuclear deterrent. Using its sophisticated sonar, the Akula would be able to track Vanguards and potentially sink them before they could launch their Trident D4 missiles.
The Daily Telegraph has learnt that, within the past six months, a Russian Akula entered the North Atlantic and attempted to track a Vanguard. The incident has remained secret until now.
It is understood that the Russians stood off Faslane, where the British nuclear force is based, and waited for a Trident-carrying boat to come out for its three-month patrol to provide the Continuous At Sea Deterrent.
While patrolling in the North Atlantic, there are a limited number of places the Vanguard is permitted to go and it is thought that the Akula attempted to track it on several occasions.
Navy commanders are understood to have ordered a Trafalgar-class hunter-killer submarine to protect the Vanguard. A recording of the Akula was made by the Trafalgar submarine's sonar operators and has been played to The Daily Telegraph.
"The Russians have been playing games with us, the Americans and French in the North Atlantic," a senior Navy commander said.
"We have put a lot of resources into protecting Trident because we cannot afford by any stretch to let the Russians learn the acoustic profile of one of our bombers as that would compromise the deterrent."