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Woman who was key wartime code-breaker commemorated with blue plaque

The design of the Joan Clarke national blue plaque (English Heritage/PA)
The design of the Joan Clarke national blue plaque (English Heritage/PA)

A code-breaker who played a key part in decrypting German messages during the Second World War will be commemorated with a blue plaque.

Joan Clarke, who was portrayed by Keira Knightley in the 2014 film The Imitation Game, was a talented mathematician who worked alongside Alan Turing at Bletchley Park, the wartime headquarters of the Government Code and Cipher School (which is now known as GCHQ).

The plaque will be installed on Wednesday at her childhood home of 193 Rosendale Road in West Dulwich, south London.

Bletchley Park
Bletchley Park, the wartime headquarters of the Government Code and Cipher School (PA)

Dr Susan Skedd, Blue Plaques historian at English Heritage, said: “Joan Clarke spent her formative years in this house, during which time she went from being a promising student, with a love of mathematics to becoming a remarkable code-breaker at Bletchley Park and later at GCHQ.”

Ms Clarke won a scholarship to study maths at Newnham College, Cambridge in 1936.

After taking the job at Bletchley Park in 1940, she was part of a “brilliant” team of cryptanalysts who decoded “more than a million” German messages which helped Allied ships avoid German U-boats, according to Dr Skedd.

Under the leadership of Mr Turing, Ms Clarke’s Hut 8 team developed methods to read all German naval communications – enabling them to reduce the number of Allied ships destroyed by the German navy from around 100 a month down to only two by November 1941.

She was personally responsible for cracking a tougher new code used by the Germans in 1942 by deducing that their Enigma machine’s new fourth rotor used the same cipher as the three-rotor system, allowing her team to continue deciphering German communications.

In the run-up to the D-Day landings, Hut 8 worked to decode the weather signals sent by the Germans and support Allied bombing raids preparing for the invasion on June 6 1944.

Dr Skedd said: “Eighty years ago this week, the work that so enthralled Joan would have been dramatically intensifying, as the Allied forces planned the D-Day invasions.”

Ms Clarke remained at GCHQ after the war and was the longest-serving member of Hut 8 and the most senior of the few female cryptanalysts at Bletchley Park.

When she left GCHQ for the final time in 1982, she promised to adhere to the Official Secrets Act for the rest of her life.

The 150-year-old London Blue Plaques scheme celebrates the link between historical figures and the buildings in which they lived and worked.

The scheme is run by English Heritage who have appealed for more suggestions of notable women.