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At the heart of this tale of betrayal were two love-struck British spies, a KGB operative masquerading as a Canadian businessman, and two American communists posing as antiquarian book dealers.

Operating between 1953 and 1961 from within the United Kingdom’s Underwater Weapons Establishment, the Portland Spy Ring sent highly classified research documents, including on nuclear submarine technology to the UK’s Cold War adversary the Soviet Union.

If only MI5 had paid more attention to the red flags and heeded the direct warnings, they could have staunched the flow of secrets to the Soviets years earlier.

It would take a tip-off from a Polish intelligence officer and one of the CIA’s most prized agents in place to bring the entire house of cards that was the Portland Spy Ring tumbling down.