1938 - 1945

Bletchley Park: Home of the Codebreakers

Bletchley Park

HM The Queen visited Bletchley Park on 15 July 2011. She said, ‘It is impossible to overstate the deep sense of admiration, gratitude and national debt that we owe to all those men and, especially, women. They were called to this place in the greatest of secrecy – so much so that some of their families will never know the full extent of their contribution.’ 

During World War I Britain built up a significant Signals Intelligence (Sigint) operation, listening to enemy radio traffic. The Government Code & Cypher School (GC&CS) was created at the end of the war and developed over the next two decades. 

Bletchley Park housed the British codebreaking operation during World War II and was the birthplace of modern computing.

Historians estimate that the Codebreakers’ efforts shortened the war by up to two years, saving countless lives. At its peak, around ten thousand people worked at Bletchley Park and its associated outstations. 

In 1938 Bletchley Park was bought by the Head of MI6. 

In August, a delegation from MI6 and the Government Code and Cypher School (GC&CS) spent around a month at Bletchley Park. 

To disguise their true identity, this delegation masqueraded as 'Captain Ridley's Shooting Party' .

By 1939 veteran cryptanalysts from World War I plus linguists and classicists such as John Tiltman, Dilly Knox, Hugh Foss and Frank Birch formed the core of GC&CS’s expertise. They were joined by men and women recruited from industry and other branches of academia.

Bletchley Park staff worked on an 8-hour shift-system: 8 am to 4 pm (days), 4 pm to midnight (evenings), and midnight to 8 am (nights). However, it was not all work - The Bletchley Park Recreational Club included a library, drama group, music and choral societies as well as bridge, chess, fencing and Scottish dancing.

Many romances blossomed here and numerous couples went on to marry. However, they had all signed the Official Secrets Act and kept their vow of silence until the story of what was achieved here began to emerge in the 1970s. Even now, some Veterans remain tight-lipped about their part in the codebreaking operation because they had been sworn to secrecy.

ENIGMA, an electro-mechanical cypher machine, was adapted for use by the Wehrmacht (German Armed Forces) and became the most widely used German encrypting device in WWII. 

Ultimately, Enigma cypher machines were used by all three branches of the Wehrmacht: Heeres (Army), Kriegsmarine (Navy) and Luftwaffe (Air-Force). Enigma was also used by the Japanese and Italians to encode secret messages.

Enigma uses rotors to scramble messages into unintelligible cyphertext. The Wehrmacht (German Armed Forces) adapted an early commercial version, marketed to the banking industry, and believed it to be impenetrable.

Each one of the machine’s billions of possible combinations generated completely different cyphertext. Finding the Key settings for each network – which were reset at midnight every day – was the challenge faced by the Codebreakers. 

The standard 3 rotor Enigma was capable of being set to approximately 159,000,000,000,000,000,000 possible combinations.

Further and increasingly more complicated versions of Enigma were developed during the war, including one used specifically by the Kriegsmarine (German Navy). 

The first break into Naval Enigma – codenamed Dolphin – early in 1941 had a significant impact on the Battle of the Atlantic. Information decrypted in Hut 8 helped to reduce the destruction wrought by the U-Boat wolfpacks in the Atlantic.

THE HUTS: As recruitment continued, conditions in the Mansion soon became overcrowded, with more office accommodation urgently needed.

From October 1939, building work began on the now famous wooden huts, which were to be the scene of some of the most extraordinary stories of the war. 

These huts worked in pairs: Huts 3 & 6 and Huts 4 & 8.

Roof repairs to Hut 3.

Bomb walls provided protection in case of air raid attack but created dark working conditions.

Hut 6 was built in January 1940 for the decryption of Enigma messages sent by the German Heers (Army) and Luftwaffe (Air Force). Hut 6 Codebreakers utilized perforated sheets and later Bombe machines to aid with this decryption.  

Decrypted messages were passed next door to Hut 3 for translation and analysis. The translators in Hut 3 had to make German military language, strictly formatted and littered with jargon, read like a credible report from a fake spy.

Hut 8 was built in January 1940 for the decryption of raw material from the Navy. Under its heads, Alan Turing and then Hugh Alexander, Hut 8, like Hut 6, became a major driving force in the development of analytical machines to speed up the decryption process. 

By early 1940 Hut 4 was used for translating and analysing messages sent by the Kriegsmarine (German Navy) which had already been decrypted by Hut 8.

These two Huts provided crucial day-to-day intelligence in the desperate battles between the Allied convoys and the U-Boat wolfpacks which were determined to cut Britain’s vitally important supply lines of food and fuel from across the Atlantic.

Bletchley Park played a key role in the D-Day landings, 6th June 1944.

The Double Cross (XX) deception, codenamed Operation Fortitude South, led the German High Command to believe that the Allied plan to invade Normandy was actually a diversion from the true target, the Pas de Calais. This deception allowed the Allies to land at Normandy while the Germans laid in fortified wait in Calais.

Operation Fortitude South was one of the most successful and important deception operations employed during the war.

“Action this day! Make sure they have all they want on extreme priority and report to me that this has been done".

Sir Winston Churchill

In 1941 a number of Codebreakers wrote to Winston Churchill in order to request more resources to ensure that the vital codebreaking work was able to match demand. Churchill's reply to an Aide was, “Action this day! Make sure they have all they want on extreme priority and report to me that this has been done".

As the importance of the huts grew, so did their staffing needs, so that by the end of the war, Hut 3, for example, was no longer a single wooden structure, but a whole range of locations and buildings on the Park. 

By early 1943 Bletchley Park had developed from a small community of specialist cryptographers into a vast and complex global signals intelligence factory. It hit its peak in 1944, when around ten thousand people worked at Bletchley and its associated outstations. 

The BOMBE machine was developed by Alan Turing and Gordon Welchman to speed up the breaking of Enigma, so that messages, once decyphered, were able to provide operationally relevant Intelligence.

The Bombe was inspired by the ‘bomba’, an earlier machine designed by the Polish Cypher Bureau. The Bombe helped to deduce the day’s Enigma settings, of both the rotors and the plug board, by eliminating the many incorrect possibilities. 

Hut 11 and later in March 1942 Hut 11a, were specifically built to house the Bombe machines; both huts were comprised of concrete to provide further protection to these precious machines. 

Hut 11a also became a training centre for the Women’s Royal Navy Service – WRNS, nicknamed ‘Wrens’ – who operated the machines. They recall the Huts being hot and noisy; Hut 11 was referred to as the “Hell-Hole”.


Bombes were mass produced and large outstations, such as Eastcote and Stanmore, were formed where these Bombes were utilised en-mass.  Hut 11a became the main control centre for all Bombes in the UK.

The USA also produced and operated Bombe machines.

Lorenz, an on-line cypher attachment which used the International Teleprinter Code, was specifically designed to be more complex, faster and more secure than Enigma.

 With 12 rotors the number of possible wheel settings was a staggering:

43 x 47 x 51 x 53 x 59 x 37 x 61 x 41 x 31 x 29 x 26 x 23 


LORENZ  was used by Hitler, the German High Command and German Army Field Marshals to carry high level communications. 

The Codebreakers called the machine Tunny and the coded messages Fish. 

Cracking Lorenz, like Enigma, relied on determining the starting position of the rotors. Intelligence from Lorenz provided Allied Commands with details of the highest German policy decisions and tactics. 

From early 1940 the teleprinter signals were intercepted but the Codebreakers knew nothing about the machine being used to encrypt the messages. 

The Codebreakers in the BP Testery had deduced the operation of the Lorenz without ever seeing the machine, a feat made possible by a German operator mistake.

The BLOCKS: In May 1941 the codebreaking factory had outgrown the Mansion, Cottages and wooden Huts. A programme of building more permanent brick and concrete reinforced blocks began. 

There was still a significant threat of air attack so Blocks A and B, which were built at the same time, were bomb-proofed and shrouded by trees, more being planted to break up shadows thrown by the moon.


Names of people, places, cover names, military units, radio stations and many other significant details were recorded and kept in an enormous index in Block C, punched onto cards using Hollerith machines. Clerks, mainly women, searched decyphered messages for details that might help the Codebreakers in the future, and built up a huge cross-referencing system. At its peak, two million cards per week were used.


Block E was the hub of outward communication from Bletchley Park. Messages were re-encyphered using Type X machines and transmitted to Allied headquarters. 

All that remains of Block F today is a concrete step and a patch of grass. But during World War II it was the world’s first purpose-built computer centre.

Block H was built in 1944 and housed Colossus and Heath Robinson machines.

COLOSSUS, the forerunner of the modern computer was developed to assist with the breaking of the Lorenz machine.

By 1943 the Germans had introduced complications which made it virtually impossible to break Lorenz by hand – or brain – alone.

The first machine, designed by Dr Max Newman and his team in the Newmanry, was christened Heath Robinson, after the cartoon designer of fantastic contraptions. It was a slow and unreliable machine but it worked.

Tommy Flowers, a Post Office electronics engineer, designed Colossus, the world’s first practical electronic digital and information processing machine. Colossus could read paper tape at 5,000 characters per second, the paper tape in its wheels travelling at 30 miles per hour. This meant that the huge amount of mathematical work that needed to be done to break Lorenz could be carried out in hours, rather than weeks.

The first Colossus machine arrived at Bletchley in December 1943 with a second in June 1944.

These Colossi were working in time for Eisenhower and Montgomery to be sure that Hitler had swallowed the deception plan prior to D-Day on 6 June 1944. 

There were eventually ten working Colossi at Bletchley Park.

The contributions of Bletchley Park’s Codebreakers to the outcome of World War II are now globally recognised and include:


* Location of the U-Boat packs in the Battle of the Atlantic

* Identifying the beam guidance system for German bombers

* The Mediterranean and North African campaigns, including El Alamein

* Launch and success of Operation Overlord, including breaking German Secret Service Enigma, complementing the Double Cross operation which misled Germany on the intended target for D-Day

* Helping to identify new weapons including German V weapons, jet aircraft, atomic research and new U-Boats

* Analysis of the effect of the war on the German economy

* Breaking Japanese codes

* The outcome of the war in the Pacific

‘The work here at Bletchley Park … was utterly fundamental to the survival of Britain and to the triumph of the West. I’m not actually sure that I can think of very many other places where I could say something as unequivocal as that. This is sacred ground. If this isn’t worth preserving, what is?’

Professor Richard Holmes, Military Historian


Credits: Story

Role — Eloize Shepherd, Apprentice
Role — Gillian Mason, Curator

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