I my opinion this would be Operation Barmaid.

Read my answer to the question originally asked and answered on Quora here.

The Untold Story of Britain’s Most Famous Submarine: Amazon.co.uk: Stuart Prebble: 8601404603235: Books

HMS Conqueror is Britain’s most famous submarine. It is the only sub since World War Two to have sunk an enemy ship. Conqueror’s sinking of the Argentine cruiser Belgranomade inevitable an all-out war over the future of the Falkland Islands, and sparked off one of the most controversial episodes of twentieth century politics.

The controversy was fuelled by a war-diary kept by an officer on board HMS Conqueror, and as a young TV producer in the 1980s Stuart Prebble scooped the world by locating the diary’s author and getting his story on the record. But in the course of uncovering his Falklands story, Stuart Prebble also learned a military secret which could have come straight out of a Cold War thriller. It involved the Top Secret activities of the Conqueror in the months before and after the Falklands War.

Prebble has waited for thirty years to tell his story. It is a story of incredible courage and derring-do, of men who put their lives on the line and were never allowed to tell what they had done. This story, buried under layers of official secrecy for three decades, is one of Britain’s great military success stories and can now finally be told.

Conqueror returning to the Clyde Submarine base (Faslane) after the Falklands War, 4 July 1982

Image and text — HMS Conqueror (S48) — Wikipedia

HMS Conqueror’s biggest secret: a raid on Russia

It was dark, in the early hours, and the sea was freezing as Her Majesty’s Submarine Conqueror came to periscope depth. Her captain, Christopher Wreford-Brown, had been stalking his target methodically, a hunter in pursuit of wary prey. There she was, 1,000 yards ahead, slow-moving, seemingly unaware of the submarine coming up on her tail. Gathered around Commander Wreford-Brown in the darkened operations room, officers and men waited in silence, inner tension masked by outward calm. It was 1982 and this was the real thing.

HMS Conqueror is famous, some would say notorious, for sinking the Argentinian cruiser General Belgrano. The nuclear-powered attack submarine, a type also known menacingly as a hunter-killer, that year became the first of her kind to fire in anger. The Belgrano was sent to bottom in short order, her ancient hull rent by two torpedoes: 323 men, many of them young conscripts, died. The Falklands war began in earnest that day, May 2 1982.

But the ship now in the crosswires was not the Belgrano. This was August, almost two months after the liberation of the Falklands, and on the other side of the world, in the Barents Sea, backyard of the mighty Soviet Northern Fleet. Conqueror was sailing as close to Russian territorial waters as was legally allowed — or maybe closer. Submariners, a tight-knit community, politely disdainful of their surface counterparts, joke that there are two types of naval vessel: submarines and targets. Wreford-Brown’s target was a spy trawler — an AGI in Nato parlance, meaning Auxiliary General Intelligence. Crammed with interception and detection equipment, they were a ubiquitous presence during the Cold War, shadowing Nato exercises or loitering off naval bases.

This one was special: Polish-flagged, she was pulling a device long coveted by the British and Americans, a two-mile string of hydrophones known as a towed-array sonar. It was the latest thing in Soviet submarine-detection technology and Conqueror’s job was to steal it. To do so, the bow was equipped with electronically controlled pincers, provided by the Americans, to gnaw through the three-inch-thick steel cable connecting it to the trawler. The name of this audacious exercise in piracy? Operation Barmaid.

Thirty years on, and the story of this mission, classified Top Secret to this day, is being told. It may be that the Russian government is learning for the first time the fate of what was one of its most closely guarded devices.

“This was a quite remarkable feat, a daring exploit that carried with it immense risk,” says the documentary maker Stuart Prebble, whose new book, Secrets of the Conqueror, discloses the existence of Barmaid. “When we think of the Cold War we think of Cuba and Berlin and missiles and tanks, but it was at sea, and under the sea in particular, where the East-West struggle was often at its most dangerous.

“I have known about Barmaid for nearly 30 years and two years ago I approached the Ministry of Defence and asked that its details be released under the 30-year rule. They spent eight months thinking about it and eventually came back and said no. Their final position was that, although they wouldn’t help, they wouldn’t try to stop me writing about it.”

Throughout the 1950s and 1960s the Anglo-Americans rested on their laurels, confident of their superiority in naval technology over the fledgling Soviet fleet. But as the 1970s wore on that confidence was eroded. Soviet submarines were not just becoming quieter and faster, they were able to turn the tables on their supposedly more advanced Western opponents. Submariners call it “bouncing”, the practice of creeping up on a hostile submarine before switching on active, wave-emitting sonar. The deafening ping in the earphones of the target crew tells them: “I’m here. If this was a war, you’d be dead.”

Towed-array sonar is different. It is passive and does not emit a signal. It floats at a prescribed depth, trailing behind a ship or submarine, simply listening for enemy submarines. Because the hydrophones are spaced out, they can achieve a multi-dimensional fix on a target, and are less vulnerable to noise from the host vessel. The American and British navies imagined themselves to be far ahead in this technology and were disturbed to discover that the Russians were matching them.

Had they caught up through ingenuity, or by spying?

The issue was sensitive for the British, who had been plagued by spy scandals in the post-war period. The “Portland Spy Ring” had betrayed naval secrets, as had the Admiralty clerk John Vassall. The Americans took the lead, conceiving a project to capture a towed array and discover its origins. General Dynamics, supplier of kit to the CIA, built the pincer equipment, which was installed in British submarines. But why not use the bigger US Navy?

“There are two schools of thought about that,” says Prebble, a former editor of World in Action. “The British believed they were selected because they had more skilled submariners, and exercises do seem to bear this out. British submariners tend not to play by the book to the extent that the Americans do.

“The more cynical view has it that if a British sub was caught the diplomatic fall-out would be less severe than if an American one was involved. No one wanted to provoke a superpower confrontation.”

Cutting a towed-array cable and making it look like an accidental loss was no easy task. Before Conqueror was fitted with the television-guided pincers, her sister ship HMS Churchill had tried to steam through an array to sever it from the towing ship. She was damaged and depth-charged for her pains. Conqueror made two attempts to use the pincers, in the Barents Sea and the Mediterranean, before her final attempt in August.

“When crews heard about these pincers, everybody thought it was absolutely crazy,” says Prebble. “Their use demanded the most brilliant seamanship, coming up from below into the array’s blind spot and edging towards the cutting point only a few yards from the tow ship. The pincers were designed to gnaw rather than slice cleanly to give the impression that the array had snagged on an underwater obstacle and been torn off.”

There, then, was Wreford-Brown, staring though his periscope that August night. The TV cameras were useless until a few inches from the target, so black was the Arctic water. Wreford-Brown and his officers had to fall back on mental arithmetic to calculate their distance from the target.

“That was the genius of the exercise,” says Prebble. “There is a way of approaching the blind spot that involves going deep and then coming up at an angle, literally below the vessel.”

The trawler’s propeller was feet away from Conqueror’s hull. A momentary miscalculation and a collision was inevitable. But nerves held and a connection was made. The pincer blades gnawed, and in seconds that seemed like hours the array was freed. Clamps held on to the cable as Conqueror dropped away to a safe depth, trailing the array by her side.

“Everyone in the control room was tense,” says one of those present. “We were expecting at any time that we would be discovered and were ready to run, if necessary.”

None of the crewmen who spoke to Prebble was prepared to confirm Conqueror’s position but the suspicion is that the operation took place inside Soviet territorial waters, just three miles from the coast. If discovered, the sub would have faced attack from Russian air and naval units. Once Conqueror reached a safe distance, divers were sent out to secure the array. The submarine later surfaced so that they could swim out again to haul the device aboard and bundle it in the hull.

Did the crew of the AGI know what had happened? Even if they suspected foul play it would not have been in their interests to admit it to their superiors. A sojourn in the gulag might have followed.

Immediately after Conqueror reached her base on the Clyde, the array was put on to an aircraft and sent for analysis in the United States. It is said that the name Conqueror was whispered with a certain reverence in the Pentagon for some time afterwards.

Following the sinking of the Belgrano, much speculation surrounded the disappearance of the Conqueror’s logs. The assumption in some quarters was that they had been destroyed to conceal embarrassing details about the submarine’s movements before and after the attack on the cruiser. Prebble thinks otherwise. “I believe the logs were shredded or incinerated to hide the Barents Sea operation,” he says. “This was a top-secret mission.”

The submarine arm is known as the Silent Service, partly because of its stealthy approach to warfare but also because of the secrecy attending its activities. Rarely does it receive public praise. Now, at least, we know of Operation Barmaid. The Conqueror’s crew had to celebrate their triumph in secret. Let’s hope they enjoyed a pint or two.

Read more about it by buying the book — The Untold Story of Britain’s Most Famous Submarine: Amazon.co.uk: Stuart Prebble: 8601404603235: Books

Alastair Majury resides locally in the historic Scottish city of Dunblane, and is a Principal Consultant and a Senior Regulatory Business Analyst working across the country. Alastair Majury also serves on the local council (Stirling Council) as Councillor Alastair Majury where he represents the ward of Dunblane and Bridge of Allan, topping the poll.

Alastair Majury, is also a director of Majury Change Management Ltd is a highly experienced Senior Business Analyst / Data Scientist with a proven track record of success planning, developing, implementing and delivering migrations, organisational change, regulatory, legislative, and process improvements for global financial organisations, covering Retail Banking, Investment Banking, Wealth Management, and Life & Pensions.

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As a data scientist/business analyst, Alastair Majury has worked closely with several high-profile businesses, such as BNP Paribas, National Australia Bank, Standard Life and the Royal Bank of Scotland Group.​A graduate of University of Glasgow, Alastair Majury earned his M.A. in Economics with Business Economics. Since then, Alastair has undergone several training sessions and earned multiple certifications for a variety of skills. More specifically, he has earned certifications in IAQ, risk management, resource management, and a bevy of other areas. ​Alastair Majury thoroughly enjoys his work.

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Alastair Majury

I am a Chartered Member of CISI, which is the UK’s leading securities and investment professional body. Alastair Majury resides locally in Dunblane.