A Russian submarine stalked the Royal Navy’s Carrier Strike Group (CSG) prompting a helicopter hunt for the vessel, The Telegraph can reveal.

Merlin helicopters were scrambled to search for the Russian submarine when the group was passing through the eastern Mediterranean.

The two aircraft dropped sonobuoys – special pieces of equipment designed to sink beneath the water to find submarines – to listen for its distinctive sounds after it was suspected to be monitoring HMS Queen Elizabeth, Britain’s new aircraft carrier, and escort ships.

The hunt for the submarine took place just four days after the confrontation in the Black Sea between HMS Defender, a Type-45 air defence ship, and Russian forces.

Russian submarines are known to be active in the eastern Mediterranean from the Tartus naval base on the Syrian coast.

Passive Directional Sonobuoy

Their primary mission is to lurk just off the coast of Cyprus, monitoring RAF aircraft launching from RAF Akrotiri to strike Daesh targets in Syria.

It is not known if the Russian boat – understood to be a diesel-electric Kilo-Class submarine from the Black Sea fleet – was caught unawares as it monitored British air operations, or if it was diverted specifically to spy on HMS Queen Elizabeth.

Two Merlin Mk2 submarine-hunting aircraft were launched, one from HMS Queen Elizabeth, the other from the Royal Fleet Auxiliary ship Fort Victoria, a replenishment tanker.

It is not known if the USS The Sullivans, an American destroyer, or HNLMS Evertsen, a Dutch frigate, both contributing to the CSG, took part in the mission.

The Telegraph understands at least one sonobuoy was dropped from the Merlins.

There are seven such helicopters deployed with the group, and these would likely have been operating in coordination with other anti-submarine assets in the group.

These include the two Type-23 frigates HMS Kent and HMS Richmond and the Royal Navy’s deployed hunter-killer submarine, thought to be HMS Astute or HMS Ambush.

While the MoD refused to confirm the incident, understood to have occurred on June 27, it said "robust measures" are in place to protect the CSG, which is on its first operational deployment.    

What is a sonobuoy?

Defence sources have suggested the suspected Kilo-Class diesel-electric submarine would likely have come from Moscow’s Black sea fleet.

Ryan Ramsey, a former Royal Navy submariner and commander of the world-renowned Perisher training course, said: "Submarines aim not to be detected – it stops you completing your tasks.

"Evasion in a submarine is really difficult when you’re going up against something as capable as Merlin helicopters. The UK has always been really effective at anti-submarine warfare using ships, submarines and aviation.

"When I was teaching the submarine command course, Perisher, it was the Merlins that the student captains worried about the most. I’m sure the opposition are doing the same thing."

The Carrier Strike Group

The Russian Navy is thought to operate only diesel-electric boats in the Mediterranean, preferring to keep its nuclear powered submarines – capable of long passages submerged – in the North Atlantic where they mainly hunt Britain’s nuclear deterrent.

Although older boats, diesel-electric submarines are quieter than nuclear powered vessels as they operate only on battery power when submerged.

However, they periodically need to hover near the surface of the ocean and run their diesel engines to replenish the batteries.

They use a "snort mast" to extend above the water to draw in air. This dustbin-sized piece of kit has a radar signature detectable to Merlin helicopters.  

The Merlin Mk2 fleet of helicopters entered service in 2014 as an upgrade for the original 1990’s Mk1 naval version. It is the Royal Navy’s principal airborne anti-submarine warfare capability.

There are 30 Mk 2 aircraft in the fleet and each one carries a crew of five. They can be armed with Sting-Ray Torpedoes, Mk11 Depth Charges and the M3m .50 Calibre machine gun.

A Ministry of Defence spokesperson said: "We do not comment on operational matters of this kind, but can confirm that robust measures are in place to protect HMS Queen Elizabeth and the ships of the UK Carrier Strike Group."

Since the incident in June, HMS Queen Elizabeth has led the CSG through the Suez Canal. The group is thought to be about to enter the Indian Ocean, on the next phase of its eight-month deployment to the Indo-Pacific.