More than fifty years after his death, historic MI5 files released cast new revelations on a senior New Zealand diplomat.
The life of Desmond Patrick "Paddy" Costello is shrouded in mystery and intrigue, with speculation rife as to whether he was, in fact, a top Soviet spy.
To Britain's Cold War spy hunters he was a dogged Soviet agent, passing secrets to Moscow, and possibly one of the most effective recruits of the infamous Cambridge espionage ring.
To his family and supporters, he was a brilliant linguist and senior diplomat, pursued by a McCarthyite witch hunt because of his unabashed left-wing politics.
The re-opening of his files is set to shed light on the long-running controversy.
The files, covering the 1930s onwards, reveal for the first time the extent of Britain's Security Service's belief that "Paddy" Costello was slipping secrets to Russian communists.
The New Zealand-born academic, soldier and diplomat never faced espionage charges, but the files show that MI5 tracked him for three decades.
In the documents, British spy stalkers claim he was seen with known Soviet intelligence officers, and handwriting analysts linked his wife Bella to a plot to use details of long dead children to build fake identities for KGB spies.
Based on KGB files he was identified as one of the Soviet Union's most important agents, but supporters have long believed he was the victim of a smear campaign.
Dr Richard Dunley, a records specialist at the National Archives in London, where his files are now open to view, said while he was subjected to three decades of surveillance, MI5 never conclusively established whether he engaged in espionage.
"Throughout much of his lifetime, MI5 went back and forth about whether Paddy Costello was a foreign agent, or simply a politically active intellectual."
Auckland-born Costello first came to the attention of MI5 while studying at Cambridge in the 1930s, the era when future Cambridge spies were recruited by the Soviets, for his openly left-wing views.
His political leanings cost him a teaching post at Exeter University, but did not thwart him gaining a job four years later with New Zealand's Department of External Affairs as second secretary at the Legation in Moscow.
Dr Dunley 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".
The files show Britain repeatedly tried to have Costello removed, Dr Dunley said, but New Zealand said it would not fire one of its most able diplomats without evidence.
Costello was eventually forced out in 1955 after 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.
However, suspicions hardened as the years passed.
“It’s quite clear at the end MI5 are convinced he was working directly with the Soviets,” Dr Dunley said.
But Costello’s son has dismissed the material in the files, saying the case against his parents had been debunked.
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 father was “pretty thin”.
He died in 1964 with the case against him still unresolved.