British author Sir Kingsley Amis was famous for his prodigious literary career, boisterous personality and radical political views. He was also the subject of regular monitoring by Britain's MI5 spy service, newly released documents show.

The reports about Sir Kingsley are among thousands of declassified MI5 files that Britain's National Archives released on Tuesday. The material dates back to the 1950s and 1960s, covering the early stages of the Cold War. It provides a rare insight into how MI5 gathered information on presumed communist sympathizers, as well as Soviet spies operating in the U.K., including a famous Russian spy who used a phony Canadian identity to steal plans for Britain's nuclear submarines.

Sir Kingsley's file is among dozens of personal files released and it shows how MI5 kept abreast of his movements. Sir Kingsley, who died in 1995, was best known for his book Lucky Jim and his critical analysis of the James Bond novels, which led him to write a Bond novel after the death of Ian Fleming called Colonel Sun. He was also an ardent communist in his youth and joined the Communist Party while attending Oxford University in 1941. He later renounced communism, embraced conservatism and adored Margaret Thatcher.

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The MI5 files reveal that the agency kept track of where Sir Kingsley lived and compiled reports on his activities, including noting at least one mistress. They relied on information from police departments, consular officials and operatives. One report mentions Sir Kingsley's move to the United States in 1957 to take up a year-long position at Princeton University. By then, he had become disillusioned with the Communist Party because of the Soviet Union's invasion of Hungary to crush a popular uprising. The MI5 note indicates that Sir Kingsley's past Communist Party membership had been flagged in a report to U.S. officials, but he managed to obtain a U.S. visa because of a mistake by a consular officer. The note says that British and U.S. officials discussed rejecting the visa, but decided not to because Sir Kingsley was likely to cause a public uproar that could reveal MI5's interest in him. Instead, a U.S. official contacted Sir Kingsley about his party membership and the author offered assurances that he was no longer a communist.

The documents indicate that MI5 ruled Sir Kingsley out as a security threat but still kept close tabs on him. One report describes him as "an enthusiastic new recruit of the Communist Party" while at Oxford and says that after he was called up for military service in 1944, he "was receiving regular supplies of the Daily Worker," a socialist newspaper. Another says his commanding officer once described Sir Kingsley as "young, inexperienced and inclined to take extremist views towards most aspects of life and to make controversial statements in the hope of creating an impression."

The trove of documents also provided new insights into the Portland spy ring, one of the most notorious cases of spying to hit Britain after the Second World War. It centred around Harry Houghton a British naval officer who worked at submarine base in Portland, England. Throughout the late 1950s, Mr. Hougton stole secret documents, including plans for HMS Dreadnought, Britain's first nuclear submarine, with the help of his lover, a clerk at the base named Ethel Gee. They passed the information on to Konon Molody, a Soviet spy who went by the name of Gordon Lonsdale and sold vending machines in London.

The documents show that Mr. Molody had concocted a detailed Canadian identity that seemed to baffle MI5 agents for a while. In one lengthy report, MI5 put together a biography of Mr. Lonsdale that said he was born in 1924 in Cobalt, Ont., and that his father, Jack, was a labourer, and his mother, Olga Elena, an American who had immigrated to Canada in 1931. It also mentioned that Mr. Lonsdale moved to Vancouver and then to Toronto, where he met up with some communist operatives, a couple named "Bacchus," who had indirect ties to Soviet intelligence. The profile was based on largely fictitious information, and it later emerged that Mr. Lonsdale was Mr. Molody and that he had secured a Canadian passport using a dead man's identity.

The MI5 files show that by 1960, the agency was beginning to figure out Mr. Molody's true identity and properly establish his communist connections in Canada that helped him obtain the passport. The service also managed to track how he moved money from a Swiss bank account to a Canadian bank.

MI5 agents also managed to learn a lot about the spy ring by following Mr. Molody to his meetings with Mr. Houghton, who was surprisingly careless. According to the documents, the men usually met in coffee shops and train stations, exchanging secret documents hidden in newspapers and carrying cases. While Mr. Molody tried to remain low-key and speak quietly, Mr. Houghton spoke in a loud voice, making it easy for the MI5 agents sitting nearby to write down their conversations. During one meeting, the files note, Mr. Houghton talked loudly about plans for upcoming meetings and how his expenses would be covered by Mr. Molody.

Mr. Houghton was arrested in 1961, along with Ms. Gee, Mr. Molody and a pair of Americans who were wanted for spying in the United States. They were all sentenced to 20 years in prison on espionage charges. Mr. Molody served less than four years, and was released in 1966 as part of an exchange with the Soviets for Greville Wynne, a Briton accused of spying in Russia. Mr. Molody returned to Moscow as a hero and his memoirs were eventually turned into a film. An inquiry later blamed lax security at the naval base for the security breach.