North Korea has been testing rocket engines and heat-shields for an ICBM while developing the technology to guide a missile after re-entry into the atmosphere following a lift-off.
Moon Jae-in, a 63-year-old human rights lawyer, has in recent days taken a higher public profile as he seeks to lead the liberal Democratic Party into the next election.
The BBC named the defector as veteran diplomat Thae Yong Ho, a counsellor at the North Korean embassy and deputy to the ambassador.
The firm, which operates in conjunction with a British counterpart, took the decision after considering its “business and geopolitical principles.”
The radio messages work by broadcasting strings of seemingly random numbers over shortwave signals to an agent in the field.
North Korea has come under tightening international pressure over its nuclear weapons programme, including tougher UN sanctions adopted in March backed by China, following its most recent nuclear test in January.
The British reporter had “distorted facts and realities” in his coverage, North Korean official O Ryong Il said announcing that Wingfield-Hayes was being expelled and would never be let in again.