Joan Clarke, the second world war codebreaker who was played by Keira Knightley in the 2014 film The Imitation Game, has been honoured with a commemorative blue plaque ahead of the 80th anniversary of D-day. The plaque was unveiled on Wednesday at Clarke’s childhood home in south London, English Heritage said. Clarke was a gifted mathematician who was recruited to work at Bletchley Park, the government codebreaking centre, where she was assigned to work in Hut 8 with Alan Turing. The pair later became secretly engaged but the engagement was broken off when Turing revealed he was gay. They remained friends. Under Turing’s leadership, the Hut 8 team developed cryptanalysis allowing them to read intercepted German naval communications. The intelligence they gleaned from these encrypted messages helped to markedly reduce the number of allied ships destroyed by the German navy, from about 100 a month to less than 20 in November 1941. “Joan was part of a team of brilliant cryptanalysts working in Hut 8 at Bletchley during the second world war,” said Susan Skedd, blue plaques historian at English Heritage. “They decoded more than a million intercepted German naval messages enciphered on the Enigma machines and so enabled allied ships to consistently avoid German U-boats. “Eighty years ago this week, the work that so enthralled Joan would have been dramatically intensifying, as the allied forces prepared for the D-day invasions.” The Imitation Game celebrated the work of Turing – played by Benedict Cumberbatch – and his team. Some critics said it played up the relationship between Turing and Clarke. “We did do some things together, perhaps went to the cinema and so on, but certainly, it was a surprise to me when he said … ‘Would you consider marrying me?’” Clarke said in a 1992 BBC documentary. “But although it was a surprise, I really didn’t hesitate in saying yes, and then he knelt by my chair and kissed me, though we didn’t have very much physical contact.” The next day Turing told her about his “homosexual tendencies”, and a few months later he broke off the engagement, believing the marriage would fail. In 1952, Turing was convicted of gross indecency and agreed to undergo hormonal treatment rather than go to prison. He died in 1954, and was pardoned in 2013. Clarke rose to be deputy head of Hut 8 and was its longest-serving member. She was paid £2 a week. In 1947, she was awarded an MBE for her codebreaking activities and later spent 20 years working for GCHQ. She died in 1996.