BEFORE she was catapulted into the public eye in September 1999, Melita Norwood lived in complete obscurity, a secretary who had worked at the Non-Ferrous Metals Research Association in London and was known to her neighbours as a “lovely old lady” who produced homemade jam. Yet Norwood was one of the Soviet Union’s longest-serving spies and the most important British female agent to be recruited by the KGB.

Though the Soviet Union had better-placed and more highly qualified agents, there is no doubt that Norwood’s access to technical details of Britain’s postwar nuclear deterrent made her highly prized by the Russians at a time when they were seeking to develop their own atomic bomb. The scale of her treachery has been compared to that of Burgess, Maclean, Philby and Blunt.

In 1999 Norwood was still living in the semi-detached house in Bexleyheath, southeast London, that she had bought in 1937 when she first embarked on her career in espionage. She had joined the British Non-Ferrous Metals Research Association five years earlier, at the age of 20.

In 1935 Melita Sirnis, as she was then, was recommended to the NKVD (from which the KGB eventually sprang) by Andrew Rothstein, one of the founders of the British Communist Party. Two years later she became a fully recruited agent.

For the next 40 years, working under various codenames, the last being “Hola”, Norwood passed on highly sensititive material to her controllers in the Soviet Union, significantly aiding the development of the Soviet nuclear programme in the immediate aftermath of the Second World War. It seems that she did it for ideological reasons. Norwood believed wholeheartedly in the image of the Soviet worker-peasant state, and was driven by the notion that “something” must be better than capitalism.

By the time she was identified as a spy in 1999, she was a Cold War relic. Many communists in Britain had become disillusioned with “really existing socialism” on the Soviet model long before the Eastern bloc collapsed. Norwood, however, clung to the communist credo to the end.

As an 87-year-old she told The Times that she had no regrets. “I did what I did not to make money, but to help prevent the defeat of a new system which had at great cost given ordinary people food and fares which they could afford, a good education and health service.” She added: “In the same circumstances I know that I would do the same thing again.”

Born in Pokesdown, Dorset, in 1912, Melita Sirnis was the daughter of a British mother — a member of the Co-operative Party — and a Latvian father, Alexander, an influential figure in the early days of the British Communist Party. She was known as Lettie as a young girl.

In infancy she moved with her family to a large house in Christchurch. Here her father, a linguist and bookbinder, translated smuggled work by Lenin and Trotsky, and founded and edited the weekly paper, The Southern Worker and Labour and Socialist Journal. The big house itself became a meeting place for local socialists, and was known as the Russian colony.

Alexander Sirnis died in 1919, when Lettie was six. She, her sister and mother moved to Southampton to live with her maternal aunt and uncle. After Itchen Secondary School, Norwood studied Latin and logic at Southampton University for a year and then went to London to seek work.

She worked at a Paddington bakery, and joined the Independent Labour Party. When the ILP split in 1936, she went over to the Communist Party. By this time she had begun working with the British Non-Ferrous Metals Research Association, a body co-ordinating technological research being done both by academics and private firms, and had, in 1935, married Hilary Norwood, a communist, trade unionist and mathematics teacher.

Yet her career as a Soviet spy was almost stillborn. She had links with a spy ring that was operating inside the Woolwich Arsenal, three of whose members were arrested in January 1938, tried and jailed for three months. MI5 had failed to spot the clues to her identity taken from a notebook by the communist ringleader, Percy Glading. She was consequently put “on ice” for a few months, before reactivation in May 1938.

Despite her relatively lowly position, Norwood was able to gain access to important scientific and technical intelligence, and such was her success at processing such material that the NKVD maintained contact with her in the following two years, when a purge of foreign intelligence officers led it to lose contact with many other important agents.