SEOUL, South Korea — He enjoyed a bit of tennis at the local club. He indulged in curry at an Indian restaurant in the west London neighborhood where he lived. As the No. 2 North Korean diplomat in Britain, he chaperoned a brother of his country’s reclusive leader to an Eric Clapton concert last year.

The diplomat, Thae Yong-ho, 55, seemed to embrace the trappings of a comfortable life in a capitalist capital thousands of miles from dreary North Korea, never hinting at disloyalty. He had lived in London for a decade, trusted because of his family’s impeccable legacy in North Korean history.

So it was a shock on Wednesday when South Korea announced that Mr. Thae had betrayed his hermetic homeland by becoming the most senior North Korean official to defect in nearly two decades.

How and when the diplomat had eluded his colleagues at the North Korean Embassy, who are required to monitor one another to thwart treason, was not clear. But a South Korean government spokesman, Jeong Joon-hee, said at a news conference that the diplomat had arrived recently in South Korea with his wife and family, proclaiming disillusionment with the increasingly isolated government of the North Korean leader, Kim Jong-un.