North Korea's leader Kim Jong-il has designated his youngest son to be the country's next leader, according to reports in South Korean media, BBC reported.
Two newspapers and an opposition lawmaker said South Korea's spy agency had briefed legislators on the move.
North Korean diplomats abroad were reportedly told to support Kim Jong-un after the North's 25 May nuclear test.
There has been much speculation over who would follow the ailing Mr Kim, who suffered a stroke last year.
Analysts have said the North's recent military actions, including last week's nuclear test, may have been aimed at helping Mr Kim solidify power so that he could name a successor.
The reports in the Hankook Ilbo and Dong-a Ilbo newspapers quoted unnamed members of South Korea's parliamentary intelligence committee briefed by the National Intelligence Service, although the spy agency refused to confirm the reports.
The Associated Press news agency reported that opposition legislator Park Jie-won, a member of the parliament's intelligence committee, told local radio he had been briefed by the government on the North's move.
Mr Park said the regime is "pledging allegiance to Kim Jong-un", it reported.
Little is known about Kim Jong-il's youngest son, who is thought to have been born in 1983 or early 1984.
The Dong-a Ilbo added that the North is teaching its people a song lauding Kim Jong-un - who reportedly enjoys skiing and studied English, German and French at a Swiss school.
There is no confirmed photograph of him as an adult.
Questions have also been raised over whether his late mother, a Japanese-born professional dancer called Ko Yong-hui, was Kim Jong-il's official wife or mistress.
The youngest Kim has been reported as being the son who most resembles his father.
He is also reported to have a ruthless streak and the strongest leadership skills of Kim Jong-il's three sons.
The BBC's correspondent, Chris Hogg, says it is not the first time there has been speculation that the youngest son was being groomed to succeed his father.
There were reports he had been named as his successor in January. In April the South Korean newsagency, Yonhap, said he had joined the North's powerful National Defence Commission.
Our correspondent notes that in a society that values seniority his youth could be a problem.
Who will eventually rule the nuclear-armed North has been the focus of intense media speculation since leader Mr Kim, 67, reportedly suffered a stroke last August.
The last succession was settled 20 years before the death of the Great Leader Kim Il-sung in 1994, and publicly announced at a party congress in 1980.
The reports of the naming of the next leader come amid growing international concern over the North's nuclear programme and its recent missile tests.
South Korea has deployed a high-speed patrol boat armed with missiles to its disputed western maritime border with the North.
It follows reports that the North has moved a long-range missile to a launch site on the west coast.
Meanwhile, at the end of a two-day summit, South Korean President Lee Myung-bak and leaders from the Association of South East Asian Nations (Asean) condemned North Korea's recent nuclear test and missile launches.