The Grand Duchess of the Duchess of Cambridge, Valerie Glassborow, and her twin sister Mary were among the codebreakers stationed at Bletchley Park during the Second World War at the top secret base.

The cracking team, which included the famous British mathematician Alan Turing, researchers and cryptographers was tasked with intercepting and interpreting enemy communications and breaking the German puzzle code.

Riddles resembling large typewriters were used by German air, sea and army forces to securely send messages during World War II.

It used a complex set of rotors and lights to encrypt messages by exchanging letters over a constantly changing "puzzle code". The code was finally broken in 1941 by mathematicians in Bletchley – a feat that proved to be a crucial turning point in the war.

Valerie Glassborow and her twin sister Mary worked in Hat 16 at Bletchley Park. She later married Peter Francis Middleton (pictured on her wedding day) and had four sons, Michael, Richard, Simon, and Nicholas. Michael is the father of the Duchess of Cambridge

Valerie Glassborow and her twin sister Mary worked in Hat 16 at Bletchley Park. She later married Peter Francis Middleton (pictured on her wedding day) and had four sons, Michael, Richard, Simon, and Nicholas. Michael is the father of the Duchess of Cambridge

Two women work in hut 6 in Bletchley Park, Buckinghamshire, during World War II. Cryptographers deciphered top-secret military communications between Hitler and his forces, which ultimately supported the victory of the Allied forces

Two women work in hut 6 in Bletchley Park, Buckinghamshire, during World War II. Cryptographers deciphered top-secret military communications between Hitler and his forces, which ultimately supported the victory of the Allied forces

Then a young, unmarried woman, Valerie Glassborow, was working in hat 16 on the estate, which is no longer standing.

Many of her colleagues were "normal" middle-class women like her, whose work, kept secret for almost half a century, helped change the course of the war.

Only a few pursued a career in the news services. Of the 9,000 people who worked at Bletchley Park during the Second World War, only 600 joined the still young GCHQ or other branches of intelligence services.

Among those who left the Domestic Life Service was Miss Glassborow, who married in 1946 in Adel, Yorkshire, Peter Francis Middleton.

The couple gave birth to four sons, Michael, Richard, Simon and Nicholas, in rapid succession.

Michael, the oldest, is the father of the Duchess of Cambridge and is known to be close to his mother.

Miss Glassborow died in 2006 without ever publicly speaking about her military service.

A black-and-white photo that looks out over one of the huts in the Buckinghamshire estate. Among the approximately 9,000 employees was Valerie Glassborow, the paternal grandmother of the Duchess of Cambridge

A black-and-white photo that looks out over one of the huts in the Buckinghamshire estate. Among the approximately 9,000 employees was Valerie Glassborow, the paternal grandmother of the Duchess of Cambridge

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