The most senior diplomat ever to defect from North Korea -- described as a "tough" and "sophisticated" negotiator by a former British envoy -- has abandoned his post as deputy ambassador in London.

Thae Yong Ho has arrived in South Korea along with his wife and three children.

"They are under government protection," said a South Korean spokesman. "On his reasons for defection, Minister Thae cited disgust with Kim Jong Un's regime, admiration for South Korea's free, democratic system and the future of his family."

Thae, who is about 50, spent a decade in London, serving as deputy ambassador for the last three years. One of his two sons studied for a diploma in public health at Hammersmith Hospital.

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James Hoare, a former charge d'affaires at Britain's embassy in North Korea, dealt with Thae on many occasions. He described the North Korean as a "very sophisticated" operator with an excellent command of English and Chinese. Hoare said: "He was a tough negotiator.

"He never in front of me expressed any ideological doubts."

North Korea's embassy in London is seen as a key diplomatic post which also handles relations with the European Union. In this role, Thae was a regular visitor to Brussels.

Hoare said he would be in a position to provide valuable information.

Thae is understood to have abandoned his post earlier this month. Defections from the diplomatic corps, which is carefully vetted for ideological purity, are extremely rare.