Charles Elwell, a charming and engaging member of the Security Service (MI5), was a master spycatcher who made his name inside the agency for exposing some of the most significant Soviet-era “illegals” - Russian agents living in Britain under false identities - during the fiercely competitive espionage battles of the Cold War.

A distinguished-looking man, Elwell achieved his greatest success with the crucial part he played in unmasking the Portland spy ring in the 1960s when five KGB agents, led by Konon Molody, posing as a Canadian running a jukebox leasing business under the name of Gordon Lonsdale, were arrested and charged with espionage.

It was a period when the KGB posed the gravest threat to the security of the country, trying ruthlessly to acquire military and technology secrets. The illegals were among the most dedicated and professional spies who wove “legends” around their lives to evade the snooping counter-espionage officers of MI5. Lonsdale had buried his real identity so effectively that it was several years before MI5 learnt, after he was swapped in a spy exchange with the British businessman and secret agent, Greville Wynne, in 1964, that the ubiquitous, good-looking, playboy-style “Canadian” was in fact Konon Trofimovich Molody.

Elwell's part in the smashing of the Portland spy ring began when a Russian mole, codenamed Sniper, informed the CIA that secret information was reaching Moscow from the Admiralty Underwater Weapons Establishment at Portland in Dorset. MI5 was tipped off, and the suspected spy inside the Royal Navy's secret facility was Harry Houghton, who appeared to be living above his means. He had a mistress called Ethel Gee, a filing clerk at the Portland base.

Elwell was the case officer, and it was not long before a team of MI5 watchers began to unravel the scale of the spy plot. Houghton and his mistress used to visit London once a month where they would meet a man, later identified as Gordon Lonsdale, in the Waterloo Road, and hand over a carrier bag. Houghton would receive an envelope in return. Lonsdale was followed, watched and bugged, and at one point he was tracked to an address in Ruislip, northwest London, where he was found to be staying with Peter and Helen Kroger, a New Zealand couple who ran a small antiquarian bookshop.

The Krogers were also part of the spy ring. Film and photographs of secret documents smuggled by Houghton out of the Admiralty base in Portland were transmitted to Moscow in the form of microdots hidden in the printed words of the antique books. The Krogers' house was full of espionage equipment. Peter and Helen Kroger were later identified by the Americans as Morris and Lona Cohen, wanted by the FBI in connection with a nuclear espionage case in America.

Elwell's investigation of this intricate case won the praise of the Lord Chief Justice of the time. For this, and for successes in other cases, including the uncovering of John Vassall, a clerk in the Admiralty, as a Russian spy, and Brian Linney, an electronics engineer, recruited by the Czechs, Elwell was appointed OBE in 1961.

Charles John Lister Elwell was born in 1919 and was educated at Haileybury College and St John's College, Oxford, where he read modern languages. He joined the Royal Navy in August 1940, and became a lieutenant in the Royal Naval Volunteer Reserve. He served in motor gun boats (MGBs) and eventually joined a flotilla based on the South Coast which landed agents in occupied Europe.

In March 1942, having successfuly disembarked two Special Operations Executive (SOE) agents in Holland, Elwell and a Dutch companion were unable to launch their dinghy through the surf to return to their MGB and he was taken prisoner. After an escape attempt, Elwell was sent to Colditz where he remained for the rest of the war. He was demobilised in October 1945.

During the next four years he held temporary appointments in the Foreign Office, including the posts of vice-consul in Tangier and private secretary to the British Ambassador in The Hague. His great wish was to have a career in the Foreign Office. But in 1949 he joined the Security Service. The following year he married Ann Glass, an elegant and accomplished colleague in MI5 who later joined the Information Research Department at the Foreign Office.

After a year in the overseas branch, Elwell was sent to Security Intelligence Far East, a joint MI5/MI6 organisation in Singapore which was run by the Security Service, where he served two tours, returning to London in 1955. He was then posted to the counter-espionage branch and served in several of its sections until 1965.

He was next seconded as a security adviser to the Ministry of Defence for three years, and was promoted to Assistant Secretary rank in 1966. On his return to MI5 head office he went back to counter-espionage, where he stayed until 1974. He spent his last five years at MI5 in the counter-subversion branch. It was considered that the skills that had made him a great investigator of espionage did not adapt well to this very different field, and he failed to win the promotion that his intelligence and abilities deserved.

After his retirement from MI5 in 1979, he joined the Institute for the Study of Conflict, where he wrote and researched on subversion, producing regular briefing bulletins. In 1983 he published Tracts Beyond the Times - a Brief Guide to the Communist and Revolutionary Marxist Press. His former employers at MI5 regarded his views on the threat posed by subversion as exaggerated, which was a source of frustration for him.

In retirement, he took up soft fruit farming. He also had longstanding family connections with the Black Country - he had earlier published The Iron Elwells: A Family History - and contributed many articles to The Blackcountryman that were published in 1991. He served as president of the Black Country Society in 1993-94. In 2001, with his sight failing, he went to live with one of his daughters.

His wife died in 1996. He is survived by their two sons and two daughters.

Charles Elwell, OBE, MI5 officer, was born on May 16, 1919. He died on January 11, 2008, aged 88