Most daughters keep secrets from their mothers, but not usually because they are a matter of national security.

During the Second World War, Ruth Bourne, from Birmingham, worked at the codebreaking base at Bletchley Park as one of the team helping Alan Turing crack the Germans’ Enigma code.

Having enlisted in the Women’s Royal Navy Service straight from school, she was selected for duties at the base and required to sign the Official Secrets Act. But that did not stop her mother.

Ms Bourne said: “My mum used to say: ‘All this secrecy. You can tell me; I’m your mother.’ I thought: ‘Yes, I can tell my mum; then it’ll be all over Birmingham in five minutes.’

“We had a very small Jewish community and everyone knew everything. I remember she complained to one of her friends: ‘I don’t know what she does, she won’t tell me’.”

Along with her Jewish ex-colleague Muriel Dindol, Ms Bourne is featured in The Bletchley Girls, a new book by Tessa Dunlop which focuses on the contribution of the women who worked there.

Ms Bourne, now aged 88, has spent the past 20 years as a tour guide at Bletchley, where she once worked eight-hour shifts on a bombe machine, the electro-mechanical device used in the decryption of Enigma signals.

She said: “I was aware that I was important, because we were told we were breaking German codes, but that’s all we were ever told. We didn’t know what the codes were, or the enormity of what we were doing.”

By the end of the war the machines, of which there were about 210 at the site, “were breaking codes in a few hours, and on an industrial scale. They used to say: ‘If Hitler wants to know what’s going on, he can ring up Bletchley and we’ll tell him’.”

After the war, Ms Bourne and her husband opened The Bendix Laundrette in north London. She said: “It was quite funny — as a bombe operator, you would put these drums on and watch them go round and round, and there I was as a laundrette manager, watching the machines go round and round. It was a home from home.”

Under the terms of the Official Secrets Act she was not allowed to talk about her work for 35 years, and it affected the rest of her life: “I got into the habit. Sometimes I’ve kept things secret that should’ve been told.”

Ms Dindol was 14 when she joined Bletchley, following in the footsteps of her older sister Anita. The two girls went to live in Bletchley town in 1939, when their family left London.

She was the youngest girl working at the base and had the job of copying German messages and sending them on to decoders.

She said: “I loved it. The war didn’t upset me as it would have if I was an adult. I had a good time — the people I worked with were all lovely.

Despite eating their kosher sandwiches together every day at lunch and going home together in the evenings, the two sisters never told each other what their jobs were.

“We slept in the same bedroom at home, but we never discussed it,” she said.

The were around 200 Jews who worked in various roles at Bletchley. “I met quite a few there,” said Ms Dindol. “Very clever men, all from Cambridge. They used to come have Friday night supper with us.”

Speaking wistfully, she said: “It was thrilling, I remember it so well. Those years are so imprinted on my memory.”

However, she has no plans to seeing The Imitation Game, the recently released film about any Turing and the codebreakers. “It’s not going to be true — these things never are — and I was there, so I already know what happened.”

‘The Bletchley Girls’ will be published on January 8 by Hodder & Stoughton, at £20