Former secretary Melita Norwood, 87, passed vital information about the British atom bomb to the Soviet Union in 1945, enabling it to build an exact replica within a year.

But she is as far removed from the stereotype of a sophisticated Cambridge graduate double agent as she is from the fictional Austin Powers, says journalist David Rose.

Little old lady

Vasili Mitrokhin is the former chief archivist from the KGB who revealed Mrs Norwood's past

He discovered her secret life for the BBC series Spying Game: "She literally lives quietly in south London and she potters about in her garden and she grows apples and she makes jam. She's a little old lady."

The secret life of Mrs Norwood only came to light after 1992 when a former chief archivist of the foreign intelligence branch of the KGB defected to West and brought with him a long list of names of spies.

Among those was Mrs Norwood, who had managed to get access to hugely sensitive information despite her lowly rank in the decades she worked for a scientific association.

Her husband disapproved

David Rose explains how he tracked down Mrs Norwood

She had never previously been confronted about her past. The only person in the world who knew had been her late husband - who strongly disapproved - and even her one daughter had no idea of the truth.

Mr Rose, a former Guardian journalist, who has spent 18 months working on the series said: "She had never been interviewed by MI5 because a long bureaucratic discussion had been taking place about whether she should be prosecuted. In the end it was decided that it wouldn't be in the public interest.

"And because of that nobody had ever approached her because if they had approached her when that possibility of prosecution still existed they could have tainted the evidence, they could have breached the law and wrecked such a prosecution before it got off the ground."

'Proud' of her actions

Mrs Norwood passed secrets about the British atom bomb to the Soviet Union

"In the course of a two-hour conversation, she confessed that she had passed on these secrets to the Soviet Union and she had continued passing on secrets until she left her job in the scientific association until 1972 when she retired and that she was in fact proud of it, that she believed this was in fact the right thing to do," he said.

"She was a communist. She believed the Soviet Union was, as she put it, an experiment which had given people better health and education and a good standard of living and the West wanted to destroy it.

"And she thought it was unfair that America and Britain were developing this new deadly weapon without giving the Soviets the chance to have it too."

Clearly, Mrs Norwood's actions had the potential to cause disastrous conflict, but as Mr Rose points out, they equally could have prevented a nuclear war.

"The Americans might well have used the nuclear weapon in the Korean war in the early 1950s if they had not known that the Soviets would then zap them with one of their own."

Decorated by the KGB

But when you speak to the octogenarian spy of the century now she gives the impression none of this really troubled her, he says.

"She's not a sophisticated animal. She hasn't got a thought out political position about whether she saved the world or might have risked destroying it. She tries to shrug off the importance of what she did. There is no doubt that she was a very important spy.

"But this is a lady who successfully practised deception for decades. She joined the Communist Party in secret in the 1930s. No-one ever knew she was a communist. Of course, no-one knew she was a spy, although she was one for years. She was a very important spy.

"She received the KGB's highest decoration, the Order of the Red Banner. She says she had no idea how important it was, but you don't know if she is telling the truth or not, or if she is deceiving herself in the way she has deceived so many people in the past."

Still, if she never worried about the geo-political implications of her actions, surely she worried about her own fate if it was ever discovered what she was doing?

"She said that she had considered the possibility when she was spying that she would be caught and she knew that the consequences would be very serious if she was.

"But, I suppose, she was just a secretary, she was in a fairly low job and nobody suspected her.

"She's a very nice lady, I have to say. No-one ever had an inkling."