SEOUL Besides being the deputy ambassador, North Korea's number two diplomat in London was the man appointed to spy on embassy colleagues and report signs of disloyalty to the feared secret police.
But last August, it was Thae Yong Ho himself who defected.
He said in an interview in Seoul that one of his jobs was to report to the "bowibu", North Korea's Stasi-like State Security Department, on everyone in the embassy, including the ambassador. But he told his embassy colleagues about the reports and made sure they were positive.
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He did not arouse suspicion himself, so it was easy to defect with his wife and two sons, Thae said.
Thae and his family disappeared from the embassy quarters in the west London suburb of Gunnersbury in August and resurfaced several weeks later in South Korea.
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"In the London embassy, I was in charge of this kind of surveillance," the 54-year-old said.
"I had to write back if they had any ideological changes or if they met any British or South Koreans in secret," Thae said of his colleagues. "But I always reported good things".
Thae has addressed a news conference, participated in chat shows and spoken in interviews since he emerged in South Korea but has divulged little about how he arranged to disappear from the London suburbs, where he was a well-liked and affable member of his local tennis club.
He says he is reluctant to share such information in case others like him, working for the North Korean state across Europe, need to hatch a similar plan.
He however said he and his family left the embassy on foot.
"I looked back at my embassy every five minutes. I stopped and looked back, and stopped and looked back," said Thae.
"That was the moment I said goodbye to my past life."
Thae is the most senior official to have fled North Korea and entered public life in the South since the 1997 defection of Hwang Jang Yop, the brains behind the North's governing "Juche" ideology, which combines Marxism and extreme nationalism.
Thae first came to London as a North Korean diplomat in 2004, when he spent four years as counsellor under ambassador Ri Yong Ho, now North Korea's foreign minister. His two sons went to local London schools, but returned with Thae and his wife to Pyongyang after his first posting there.
Thae said he had always had his doubts about the direction he felt North Korea was going in, but it wasn't until the 2011 death of Kim Jong Il and the subsequent handover of power to his son Kim Jong Un that he felt things were starting to unravel.
He has said the North Korean elite are outwardly expressing their discontent towards Kim Jong Un and his government as more outside information trickles into the isolated country.
In 2013, Thae returned to London with his family, the same year Jang Song Thaek, Kim Jong Un's powerful uncle by marriage, was executed in a brutal purge that included extended members of Jang's family and business contacts.
"It was a huge nationwide purge," Thae said, adding it prompted him to plan an escape.
"I had to leave the system".
(Editing by Raju Gopalakrishnan)