After several days of rumours, South Korea's Unification Ministry confirmed that Thae, who is thought to be in his late 50s, was now in Seoul with his family.
"They are now under the Seoul Government's protection and relevant institutions are proceeding with necessary procedures," Jeong Joon Hee, a ministry spokesman, told reporters, according to the Yonhap News Agency.
Defectors who held senior political or military positions within North Korea are extensively debriefed by the South Korean intelligence agency, and then offered to United States military intelligence. They generally do not go through the kind of resettlement programme for regular defectors - where they learn things like how to use a credit card and the internet - but often end up at a government-linked think-tank.
The South Korean Government, which has been taking a suddenly tough approach to North Korea since its nuclear test in January, used Thae's escape to take another swipe at the regime in Pyongyang.
"This case shows that North Korean elites view that there is no hope in their country," said Jeong of the unification ministry. "It also indicates that North Korea's regime's internal solidarity is weakening."
But some analysts speculated that Thae's departure could be linked to tougher sanctions against North Korea following this year's nuclear and long-range missile tests. North Korea's embassies are thought to be money-making centres and over the years diplomats have been caught smuggling everything from gold and cigarettes to rhino horns and heroin. Increased scrutiny of North Korea's activities, legal and otherwise, could make it harder for diplomats to meet their quotas.
South Korean officials did not disclose how or when Thae arrived, but the Guardian, quoting a fellow student at his son's school, suggested the family had "disappeared" sometime in July. Thae had talked publicly about living in London with his wife and that his son attended high school in Acton, in west London. He has also mentioned an older son who had graduated from a university with a degree in medicine or public health.
North Korean diplomats generally must leave one member of their immediate family in Pyongyang - the regime's insurance against defections - and it was not clear if Thae had managed to take all of his family with him.
Thae's defection from London could complicate the delicate diplomatic ties between London and Pyongyang. A spokeswoman for the British Foreign Office said, "It's not a story we're commenting on."
Thae was known in London for attending political and cultural events and, at times, seemed good-natured, even humorous, as he joked in English about the high cost of life in capitalist London.
FLEEING NORTH KOREA
1 Thae becomes the most senior North Korean diplomat to defect since the Ambassador to Egypt sought asylum in the United States in 1997.
2 In April, the South Koreans announced the arrival of a colonel from North Korea's Reconnaissance General Bureau, the primary spy agency and the department believed to be behind the hacking of Sony Pictures in 2014 and the sinking of a South Korean naval corvette in 2010.
3 That same month, South Korea confirmed that 13 North Koreans working at a state-run restaurant in China - another key source of foreign currency for the regime - had defected.
Warwick Morris, a former British ambassador to South Korea who had met Thae on about four occasions, said that the diplomat was "smooth and sophisticated in a slightly North Korean kind of way".
But Thae also displayed the particular brand of public devotion to his Government shown by North Korean diplomats. In 2014, for instance, he scolded British journalists during a speech at a London bookstore for allegedly exaggerating the security level at a major event in Pyongyang, comparing it to what reporters might face if attending an event at Buckingham Palace.
"There has been so much ideological work by the ruling class of the British," he said, according to a video of his speech posted on YouTube. He added that they have "brainwashed" the working class.
John Nilsson-Wright, head of the Asia programme at Chatham House, a London-based think-tank, said Thae was "very able, dapper, spoke excellent English. . . he was atypical in a world of faceless bureaucrats and had a genuine interest in the country he was based in.
"I was surprised to learn of his defection, but not totally," Nilsson-Wright said. "Anyone who is as bright as he is can see the difference between the official lines of the Government and the reality of the outside world."
Analysts agreed that Thae's defection could be highly valuable to South Korea and the West. Thae would have come into contact, Nilsson-Wright said, with a number of influential people in the current North Korean administration. "He will have good details of how the government works," he said.
North Korea allows only citizens deemed most loyal to the regime to travel abroad, so Thae's fleeing marks the latest in a series of embarrassing defections.
