Lieutenant Commander David Balme: Leader of the operation which captured the Enigma cipher machine and helped win the Second World War

The discovery allowed Bletchley Park codebreakers to penetrate the German cipher called Hydra

Balme: for a time he was the Royal Navy's youngest Lieutenant-Commander

On 9 May 1941, Sub-Lieutenant David Balme shouted the order to lower the ship's sea-boat into the swelling mid-Atlantic. Three hundred yards across the waves, there wallowed his destination, a stricken German U-boat, stern down.

Balme and his men were from the British destroyer Bulldog, the leading ship in the 3rd Escort Group accompanying convoy OB 318. Her fellow escorting vessel, the corvette HMS Aubretia, had forced the U-boat to surface with depth charges, and Bulldog's gunfire had damaged her. She had been abandoned by her crew. Balme's armed boarding party, which rowed across, had orders to strip her of anything useful.

They arrived soon after midday to windward of her. Balme clambered up her curved, slippery surface, and, revolver at the ready, mounted the fixed ladder of the 12ft conning tower. Going down inside, he had two hatches and more ladders to negotiate. It meant replacing the weapon in its holster to grip with both hands, and descend bottom-first. If any Nazi crewman had stayed on board, he thought, I'm an easy target.

An eerie blue light bathed the U-boat's nerve centre in the chamber below, an array of unfamiliar wheels and dials. A hissing came from somewhere, and he could hear the ocean slosh against the hull. There might be booby traps; there might be scuttling charges set to explode. He went up to the bow: nothing; the stern, too, was empty.

He formed his men into a chain to pass out books and documents. They included a stoker, Cyril Lee, and a telegraphist, Allen Long. The stoker's job, to restart the engines, proved too risky, but the telegraphist at once told Balme: “This looks like an interesting bit of equipment, Sir.” It resembled a typewriter, but lit up strangely when Long pressed the keys. It was a German naval “Enigma” cipher machine. The party found daily settings and procedures for its use. Written in soluble ink, they risked being lost if dropped in the sea, but, Balme recalled: “nothing even got wet.”

It took him and the party, Able Seamen Sidney Pearce, Cyril Dolley, Richard Roe, Claude Wileman, Arnold Hargreaves and John Trotter, six hours to clear the U-boat. Bulldog's commanding officer, Lieutenant-Commander Joe Baker-Cresswell, who had refrained from ramming the U-boat in order to seek out its secrets, “very kindly sent some sandwiches over”. Balme ate his at the desk of the U-boat captain. This was the now-deceased Kapitänleutnant Fritz-Julius Lemp, who after sinking two ships from the convoy, feared that Bulldog or her fellow destroyer HMS Broadway must smash his hull, and, hurling himself into the sea, drowned with 14 of his men. The rest were taken prisoner.

In the afternoon, Balme had his semaphore flagman call for Bulldog's chief engineer officer, GE Dodds, who sought to increase the U-boat's buoyancy. At 6.30pm Baker-Cresswell ordered them back to Bulldog, and took U-110 into tow. She sank next day, before Bulldog reached Iceland to refuel.

The boarding, called Operation Primrose, drew a signal from the First Sea Lord, Admiral Sir Dudley Pound: “Hearty congratulations. The petals of your flower are of rare beauty.”

King George VI called the find “perhaps the most important single event in the whole war at sea.” Balme received the Distinguished Service Cross. The need for secrecy, the King told him, precluded the higher award he would have preferred to make. The others were Mentioned in Despatches, and Baker-Cresswell received the Distinguished Service Order.

The machine and documents helped turn looming defeat in the Battle of the Atlantic to victory. For much of the war's first two years, when Britain stood alone, U-boat attacks sank too many ships bringing vital supplies. “The only thing that ever really frightened me during the war was the U-boat peril,” Winston Churchill recorded. Baker-Cresswell, years later, wrote to Balme: “The whole beauty of our exploit was the providential timing of it. The situation was just about desperate… If losses in the Atlantic had gone on increasing at the same rate… we would have had to sue for peace.”

The discovery allowed Bletchley Park codebreakers to penetrate the German cipher called Hydra. Charts from U-110 showing German minefields and swept channels made possible the successful St Nazaire raid of March 1942.

Balme later joined the Fleet Air Arm as a navigator with No 826 Naval Air Squadron. He was Mentioned in Despatches for dropping flares to guide Wellington bombers before the second battle of El Alamein in October 1942. On promotion he was, for a time, the Royal Navy's youngest Lieutenant-Commander.

His career began on the cruiser HMS London, then the cruiser HMS Shropshire. He served in the minesweeper HMS Sutton, and on war's outbreak was in the destroyer HMS Ivanhoe. He later served in the cruiser HMS Berwick and the battle-cruiser HMS Renown.

The second of five siblings, he had three brothers and a sister. The family's wool broking business was founded by Charles Balme in 1820. He spent his early childhood in Wimbledon, before going to Britannia Royal Naval College, Dartmouth, at 13. In 1947 at Winchester Cathedral, he married Susan im Thurn, a former Wren. He left the Navy in 1949 and joined the family business, becoming chairman of the City of London Wool Brokers in 1970.

The U-110 story emerged only in the 1970s. There was anger in Britain when, in 2000, the American director Jonathan Mostow made a film starring Harvey Keitel and Jon Bon Jovi which renumbered the U-boat “571” and pretended that US sailors had made the find. Balme reflected that before Hollywood made the film, no one had heard of the episode.

MPs nevertheless tabled a motion in Parliament regretting that Hollywood had chosen to distort the truth, and on 7 June 2000 the Prime Minister, Tony Blair, agreed with Brian Jenkins, MP for Tamworth, that the film was an affront to the memory of British sailors who had died.

The Royal Navy, unusually, accorded Balme a special tribute on his death: “The naval family has lost a quiet hero whose actions helped change the course of the Battle of the Atlantic – and World War II.”

David Edward Balme, naval officer and wool broker: born London 1 October 1920; DSC 1941; married 1947 Susan im Thurn (one daughter, two sons); died Lymington, Hampshire 3 January 2016.