Code-breaker who kept war secrets until world knew
While raising her family in post-war Edinburgh, she spoke not a single word about the years she spent cracking enemy codes at British intelligence headquarters at Bletchley Park.
Only now, as the world of the Second World War code-breakers is depicted in the film Enigma, has Mrs Mitchell revealed that she was part of a secret army which deciphered Nazi messages and helped the Allies to win the war.
After attending a special preview of the film starring Kate Winslet and the Scottish actor Dougray Scott as code-breakers at the Buckinghamshire intelligence unit, Mrs Mitchell revealed why she finally feels able to break her promise to take the secret of Enigma to her grave.
The 78-year-old grandmother said: "The first time I ever remember speaking about Enigma was when I saw that another code-breaker had written a book about his experience. I said to [my husband] Angus, ‘he can’t write about that. We all signed the Official Secrets Act.’
"Then suddenly more books came out from other code-breakers and while on one hand it seemed all right to speak about Enigma, part of me still felt we might all get into trouble. Only now do I really feel that the veil of secrecy has been lifted and we are finally getting recognition for what we did."
Mrs Mitchell was 20-year-old Ann Williamson when she was recruited to work at the top-secret Buckinghamshire establishment after studying maths at Oxford University.
For more than two years she worked in Enigma Hut 6, as part of the British intelligence effort to decipher the secret code used by the Nazis to transmit intelligence about troop movements.
Dubbed by the then prime minister Winston Churchill as his "Golden Geese", the code-breakers methodically typed possible code combinations into specially designed machines similar to modern calculators.
Each night the young workers would first have to break the Nazis’ red code. The race was then on to decipher the yellow and green codes .
At the stroke of midnight each night, the battle to crack the red code recommenced, as the Germans switched their messaging system daily in an attempt to foil British intelligence.
The discovery of the Enigma code has been credited with shortening the war by at least two years, often allowing the British government to know where German troops were moving to before the soldiers were aware of it themselves.
But such was the nature of Mrs Mitchell’s task, that she had no day-to-day indication of how significant her achievements were.
She said: "We knew we were cracking Hitler’s codes and we knew we were doing well as every day we cracked every new code, but we never really knew what direct effect our work was having.
"We never knew what was in the full messages as we were always too busy moving on to the next code.
"We just knew we had to battle against the clock."
In 1945, on the day the war ended, Mrs Mitchell was told her services were no longer required and was instructed to forget about Enigma and her work at Bletchley Park.
For more than 50 years, she honoured the commitment, telling her family that she simply worked for the Ministry of Defence.
Even when her husband, Angus, regaled their four children with his exploits in the 1944 Normandy landings, she did not reveal her own war-time role.
Mr Mitchell, who was a senior civil servant in the Scottish Office, said that while surprised at his wife’s revelation, he had always known she was capable of remarkable things.
He said: "When the war ended no-one really spoke about what they did.
"It was quite a surprise to learn that she worked on Enigma, but I always knew she was a highly intelligent woman so I wasn’t shocked."