Thae Yong Ho, who served as deputy to the ambassador, was responsible for promoting the image of his country to British audiences, BBC news reported on Wednesday.
He lived in Britain with his family for 10 years and disappeared from his home in west London several weeks ago.
"A North Korean diplomat in London is going through procedures to seek asylum in a third country," South Korean media reported.
"The North Korean embassy made belated attempts to figure out the diplomat's whereabouts, but failed," it said.
"A high-level defection, if confirmed, will be deeply embarrassing for the regime," the report said.
"London has always been an important diplomatic priority for North Korea given the amount of personnel stationed there and the considerable resources devoted by the regime to maintaining its presence there," it said.
Neither the Foreign Office nor the embassy has commented yet on the development.
