Female codebreakers: the women of Bletchley Park

His outstanding role in the creation of the bombe machine, an electromechanical testing device essential for unravelling German Enigma encoded messages, was hugely significant. It is a feat perhaps only rivalled by that of Tommy Flowers, the engineer who designed the even more advanced Colossus, the world’s first programmable computer.

The need to outsmart one’s enemy frequently led to ground-breaking innovations during WW2, yet the women who worked at Bletchley have often been overlooked in this story because, with the exception of three or four female cryptanalysts, the vast majority of top-end codebreakers during the war were men.

The story of how girl power – often school-girl power – turned what began as an eccentric experiment into the world’s most impressive codebreaking factory is less well known, but no less important.

• Who worked at Britain's wartime intelligence factory

A question of trust

In 1938, with war on the horizon, GC&CS temporarily moved out of London, to avoid bombing raids, but by August 1939, its secret home in Bletchley Park had become permanent. Initially, this fledging operation was staffed by just 186 people. ‘Men of a professor type’, particularly mathematicians were targeted, with early recruits including Turing, Gordon Welchman and Alfred Dillwyn Knox. Their work, however, was supported by an expanding team of chiefly civilian women.

Perhaps inevitably at the beginning of the war, Establishment Britain recruited from their own when it came to Bletchley’s secret operations. The German military must never know that Britain was in the process of achieving what Hitler believed to be impossible – the decoding of Enigma. It was imperative that those selected could be trusted to keep quiet, and from kinship springs trust. Many of the first women at Bletchley Park came from ruling-class families who knew each other.

• Listen | Tessa Dunlop shares the stories of women who worked at Britain’s codebreaking centre in World War Two

Lady Jean was a 19-year-old Scottish aristocrat and debutante. She had been tipped off about a “hush, hush mission at Bletchley Park” by her father’s friend, Lord Mountbatten. Actress Pamela Rose (née Gibson), received a letter from an “interfering godmother”. If girls’ education was rarely a priority between the wars, an upper-class focus on being accomplished and attending foreign finishing schools certainly had its advantages. Pamela’s understanding of German, for example, was deemed to be useful for filing decrypted messages.

For that work she sacrificed her first role on the West End.

From the outset, Bletchley, aka Station X, could not operate without the constant ‘traffic’ of German Enigma messages. This came from the Y Service, Bletchley’s vital other half, with its numerous listening stations intercepting German radio communications. Among the staff of the Y Service were members of the Women’s Royal Naval Service, or ‘Wrens’. One of these was 18-year-old Pat Davies (née Owtram). Assigned as a Special Duties Linguist after an intense training course, her first post on a “highly secret mission” was at Withernsea, Yorkshire. “As soon as one of the German ships came up, you wrote down exactly what you heard,” she later recalled.

A lot of what Pat remembers was the meaningless clumps of letters. “Anton, Bertha, Cesar... I always thought it was odd hearing the war all the time from the German side. The whole thing would be written down and then one of us would call Station X. I had no idea what Station X was.” Pat was one of many secret ‘listeners’, whose precious encoded data was being sent to the rapidly expanding team at Bletchley.

• Read more | The changing role of women in British computing

In late 1941, for the first time in British history, conscription for women was introduced. By then Bletchley was already reliant on a massive female workforce, like so many other wartime institutions. A group of the original cryptanalysts had sent a stern missive that October to Winston Churchill saying they did not have sufficient resources and staff, which saw the prime minister instantly and dramatically scale up operations. It would keep growing so that by 1944, Bletchley employed 8,743 workers, three-quarters of whom were women.

Although ‘posh’ civilian girls were first to arrive, the sheer numbers required saw the majority of staff being recruited by the military services. Among them was Charlotte Webb (née Vine-Stevens), who was 19 when she was selected from her Auxiliary Territorial Service training camp. Shifts took place in a small room in the Bletchley mansion.

“I worked on the card index, putting things into date order and registering them under their call signs. Nothing was in clear language. It was all in groups of letters or figures on sheets of paper – masses of them,” says Charlotte. Her job was not to understand, but to register every message passing across her desk.

Learn to love the Bombe

It was Turing’s development of the bombe that radically increased the rate at which Enigma could be read. By 1944, a force of 1,676 Wrens were dutifully tending more than 200 bombes – described as having the appearance of “great metal bookcases” – which were harvesting up to 18,000 Enigma messages daily. Alongside Pat’s interceptions and Charlotte’s data processing came Wren Ruth Bourne (née Henry)'s mechanical vigilance as another component on the codebreaking conveyor belt. Her job was to operate one of the bombes. Churning with around 100 rotating drums, 12 miles of wire and one million soldered connections, it made for an intimidating prospect, but what Ruth thought of her work was irrelevant. “Nice girls do what they are told,” she explains.

The bombe was just the first of the industrial sized behemoths that transformed life at the Park. Another Wren, Joanna Chorley (née Stradling), worked in the Newmanry, a section tasked with reading the highly sophisticated ‘Fish’ communications sent using the Lorenz cipher between Hitler and his high command. Up to a hundred times longer than Enigma, Fish messages were invaluable. So much so that a technological whopper was born to analyse their contents: Colossus.

Joanna remembers her first meeting with the machine, which was the size of a room: “It was ticking away, and the tapes were going around and all the valves, and I thought what an amazing machine. Like magic and science combined!” Joanna had fallen in love with the world’s first electronic computer. “It’s a bit silly really isn’t it? But I did love the beast,” she recalled.

Repetition and reward

For many of the Bletchley Girls, their roles were compartmentalised and prioritised accuracy, stamina and occasionally a foreign language. Lady Jean was disappointed with her job. Her shift work, designed to make up for a short fall of bombe machines, was frustratingly repetitive.

“I marked letters in German messages, then perforated those same marks and compared one message on top of another. If three holes were on the top of three other marked ones, these were put through the hatch to the next room. Doing this for a year sent me nearly crazy.”

Exceptional moments were keenly savoured. Rozanne Colchester (née Medhurst) was a teenager who could speak Italian courtesy of a childhood spent in Rome. Her basic decoding was formulaic and the information revealed fairly dull, but late one night “after many trials and errors I found myself faced with a message that made sense”. In her small work room in Buckinghamshire, she read something no one else in the Allied forces knew – in three and a half hours, Italy’s SM.79 torpedo bombers and SM.82 transport carriers would leave Tripoli and head across the Mediterranean. With the Desert War over by June 1943, the crippled Italians were making for Sicily, but thanks to Rozanne they never reached their destination. The information she read was radioed to the RAF in North Africa. “Very soon our aeroplanes were in the air and all the Italian aircraft were shot down!”

Few experienced such stand-out moments, but all of the women remember the onus placed on secrecy. No one could forget their introduction to the Official Secrets Act. Charlotte recalls a vast document she was forced to read on the spot. Ruth was told if she broke her oath of secrecy, she would go to prison “at the very least”. Joanna was under no illusions, either: “We knew damn well what will happen if we blab. It will kill people. We knew we couldn’t talk for a reason.”

During his only visit to the Park in 1941, Winston Churchill described Bletchley as “the goose that laid the golden egg but never cackled”. And everyone kept their secrets for more than 30 years. Bletchley’s impact on the war is now common knowledge, but before the 1970s some Bletchley Girls weren’t even aware they had been involved in codebreaking. Pat was one of them.

“It was surprising to hear that Bletchley had achieved such great things,” she says, “because at the time we never got any feedback. We didn’t know how important we had been.”

Tessa Dunlop is a historian and broadcaster

This article first appeared in the July 2019 issue of BBC History Revealed