The 87-year-old was revealed as one of the Soviet Union's greatest Cold War assets last September, surrounding the publication of The Mitrokhin Archive, a book based on notes taken by a senior KGB archivist and smuggled out of Russia following his defection.
The details contained within the book were staggering. In the final months of the Second World War, this now frail woman had passed to the Russians, secrets that had apparently helped them build the atom bomb.
"I thought it was an experiment, what they were doing out there - a good experiment and I agreed with it," said Mrs Norwood, when questioned by reporters back in September. "I did what I did because I expected them to be attacked again once the war was over. Chamberlain wanted them attacked in 1939 - he certainly expected Hitler to go East.
"I thought that they should be adequately defended because everyone was against them and because they had been through such hardship from the Germans. In the war the Russians were on our side, and it was unfair to them that they shouldn't be able to defend their weaponry."
Mrs Norwood had access to the vital information through her job at the British Non-Ferrous Metals Research Association in Euston, London, where as a secretary since 1937 she was aware of certain information about metals being used in the development of Britain's atomic weapons.
In 1951 there were suspicions about her loyalty to Britain and, as a result, her security clearance that gave her access to such secrets was revoked. And yet, it seems, Mrs Norwood, happily married to a maths teacher, continued spying for the Russians for another 27 years.
Mrs Norwood has questioned the importance of the information she passed on. She said she was never under any pressure and could not remember the names of the various Russians she had dealt with over the years.
And yet Col Vasili Nikitich Mitrokhin, who defected to the West in 1992 after contacting the British Secret Intelligence Service (MI6) in Latvia, described her as one of Russia's most important female spies of the Cold War. Her files described her as "exceptionally reliable" and she eventually received the KGB's highest decoration - the Order of the Red Banner.
Whatever Mrs Norwood considered the importance of the information she passed to the Russians, they were able to develop and test their own atomic weapon in 1949 - just months before an assessment by America's Central Intelligence Agency concluded it would not be able to do so until 1954.
Following the defection of Mitrokhin, the former chief archivist, and his six large trunks containing 60 volumes of files copied from the records of the KGB, details about Mrs Norwood were passed to Britain's security service, MI5.
The decision was then taken that there was little prospect of a successful prosecution being brought against Mrs Norwood. As a result she was not approached by the security service officers.
And so, at her semi-detached 1930s house decorated with CND posters, Mrs Norwood carried on living her secret, unaware that anyone apart from her Russian handlers knew of her past. Her husband, Hilary, who had known and disapproved of her spying activities, died in 1996.
That all changed rapidly following press reports and a BBC television programme based on the book by the Cambridge professor Christopher Andrew. Almost overnight, Mrs Norwood was at the centre of a flurry of activity to expose "Reds under the bed" - including those other "spies" the CPS confirmed yesterday would also not be facing prosecution.
Just who initiated the contact that gave the tall, enigmatic professor access to the files of the KGB archivist, remains unclear. The Home Secretary, Jack Straw, has asked the House of Commons Intelligence and Security Committee to investigate the matter. There are doubts about how much of this information will be made public.
In the meanwhile Melita Norwood, who twice visited Russia after her retirement, seems to stand by her actions. Although she was yesterday saying little, in September, she said given her time again she would do the same.
"It's a worldwide thing, " said Mrs Norwood, whose father was a Latvian bookbinder and mother a member of the Co-Operative Movement. "The various countries of this rotten capitalist system with its unemployment, its wars, and making money - I hope it comes to an end."