Many were women who volunteered to join the ATS or Wrens and found themselves seconded to what seemed mundane but secretive jobs.
Only after decades did they learn of the vital role they played. Joyce Aylard, 93, of north London, joined the Wrens in 1943 and was one of the 800 women who operated the codebreaking machines.
She said: “We worked in eight-hour shifts. When the machine stopped, we’d take down the numbers and give them to a partner, who operated a checking machine.
“Most of my family didn’t know what I did – just that it was something important.
“It wasn’t until 1974, with the publication of a book about Bletchley Park, that anyone learned what went on behind the scenes.”
Betty Webb, 95, of Birmingham, an ATS volunteer, said: “I only discovered two years ago that I had been handling traffic between German officers to do with the Holocaust and the movement of Jews to concentration camps. It was quite startling.”