Lessing first came to the attention of colonial-era intelligence agents in 1943 in Southern Rhodesia, now Zimbabwe, where she grew up, and from then on spies kept tabs on her in Africa and Britain until 1964.

The author of "The Golden Notebook", one of the most influential novels of the 1960s, was awarded the Nobel Prize for Literature in 2007. She died in 2013 aged 94.

MI5, Britain`s internal intelligence service, had built up a five-volume secret file on Lessing that has now been placed in the National Archives and was made public on Friday.

One of the earliest documents is an extract copied from a letter from an Air Ministry official, dated 1944, about the Left Book Club that Lessing ran in Salisbury, now Harare, with her second husband Gottfried Lessing, a German Communist.

"Most topics of discussion there usually end up in anti-British, anti-capitalist and anti-imperialist vapourings," the official wrote, noting with concern that "persons with foreign accents" mingled at the club with Royal Air Force personnel.

After her divorce from Gottfried, Lessing moved in 1949 from Rhodesia to London, where security agents picked up her trail.

In 1952, the external spy agency, MI6, took an interest in a visit she made to the Soviet Union with a delegation of left-wing British writers, providing a note kept in the MI5 file.

"Her communist sympathies have been fanned almost to the point of fanaticism owing to her upbringing in Rhodesia, which has brought out in her a deep hatred of the colour bar," the note says, referring to the restricted rights of black Africans.

"Colonial exploitation is her pet theme and she has now nearly become ... irresponsible in her statements ... saying that everything black is wonderful and that all men and all things white are vicious."

"IMMORAL PRACTICES"

Several of Lessing`s 1950s annual membership cards of the Communist Party of Great Britain feature in the file.

Such was the interest in her that agents carefully cut out a tiny ad placed by Lessing in 1955 in the Communist newspaper the Daily Worker, advertising cheap accommodation for a woman with a child who could provide company for her youngest son, Peter.

One report dated January 1956, provided to MI5 by the Special Branch of London`s Metropolitan Police, described her activist`s lifestyle and contacts in suspicious terms.

"Her flat is frequently visited by persons of various nationality, including Americans, Indians, Chinese and Negroes," the report said.

"Some of the visitors seem to stay at the flat for days at a time and some of the visits are made by apparently unmarried couples," Special Branch noted, concluding gravely: "It is possible that the flat is being used for immoral practices."

Later that year, the file documents how Lessing left the Communist Party in protest over its support for the crushing of the Hungarian uprising by Soviet forces. After that, MI5`s interest in her activities appeared to wane, although new documents were added to the file until 1964.

Looking back years later on her years as a Communist Party member, Lessing said: "I can`t understand why I was so gullible."

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