To Britain’s Cold War spy hunters, he was a determined Soviet agent and possibly one of the most effective recruits of the notorious Cambridge espionage ring.
To his family and defenders, he was a brilliant linguist and senior diplomat, pursued by a McCarthyite witch hunt because of his unashamed left-wing politics.
Now more than 50 years after his death, historic MI5 files released to the National Archives in Kew cast new light on the controversy over the life of Desmond Patrick Costello.
The files covering the 1930s onwards for the first time reveal the extent of the security services suspicions against “Paddy” Costello.
The New Zealand born academic, soldier and diplomat never faced espionage charges, but the files show that MI5 tracked him for three decades.
The documents show British spy hunters claimed he went on to be seen with known Soviet intelligence officers, while handwriting analysts had linked his wife Bella to a plot to use details of long dead children to build fake identities for KGB spies.
But Costello’s son on Tuesday dismissed the material in the files, saying the case against his parents had already been debunked.
The documents released today will provide a fascinating new insight into the man, but they are unlikely to entirely satisfy either side
Dr Richard Dunley, National Archives
Mick Costello, a former journalist and industrial organiser for the Communist Party and now an academic at the University of Kent, said the evidence presented against his parents was “pretty thin”.
His father was dogged by accusations he was a spy throughout his career and after his death was identified by the MI5 historian, Prof Christopher Andrew, based on KGB files, as one of the Soviet Union's most important agents.
Costello and his openly left-wing views first came to the attention of MI5 while he was studying at Cambridge in 1930s, in an era when Kim Philby and his spy ring were recruited by the Soviets.
His political sympathies cost him a teaching post at Exeter University, but that did not stop him getting a job four years later with New Zealand's Department of External Affairs as second secretary at the Legation in Moscow.
Dr Richard Dunley, a records specialist at the National Archives in Kew, said Costello was reputed to have informed the New Zealand prime minister he was "a little bit left wing" only to be told: "Oh well, it won't hurt us to have one or two communists in Moscow".
MI5 was horrified Costello had been appointed to such a sensitive post, but when they tried to raise their concerns with the Dominions Office, they had to admit the case against him was "a thin one".
Dr Dunley said the files showed Britain repeatedly tried to have Costello removed, but New Zealand said it would not fire one of its most able diplomats without evidence.
Costello was eventually forced out in 1955 after it became clear Britain and America were reluctant to share intelligence while he was in post.
He returned to Britain and resumed his academic career at Manchester University.
Dr Dunley said the records showed MI5 had switched back and forth during three decades tracking Costello as to whether he was a foreign agent, or “simply a politically active individual”.
Suspicions hardened as the years passed however.
“It’s quite clear at the end MI5 are convinced he was working directly with the Soviets,” Dr Dunley said.
In 1960, his wife was linked by her handwriting to a KGB operation to build false identities for Russian spies. Two years later Costello was reported to have met two Soviet intelligence officers, though Dr Dunley said the details of the meetings were “sketchy and there were even some doubts over the identification of the Russian officers”.
Costello died in 1964.
Dr Dunley said: "The debate has continued in the open since his death and the documents released today will provide a fascinating new insight into the man, but they are unlikely to entirely satisfy either side. This story appears to be one with more to run."
Costello's 80-year-old son said the allegations against his parents had been debunked by a 2007 book called The Sixth Man by Sir James McNeish.
He told the Telegraph the latest information appeared “a rerun of the sort of stuff that was previously run against my father”.
He said he had never heard of allegations against his mother before. He added: “An MI5 handwriting expert wouldn’t stand up in court anywhere.”