TOKYO - Japan's ancient monarchy was in tumult Thursday, with the imperial household insisting its ageing emperor had no plans to abdicate after reports he wanted to step aside.

Respected national broadcaster NHK - citing palace and other sources - said Wednesday that 82-year-old Akihito wanted to pass the throne to his eldest son sometime within the next few years.

Any such abdication - the first since 1817 - would be a severe jolt to a country where the 2,600-year-old royal family symbolises stability and continuity.

Media watchers say NHK and Kyodo News, which separately carried a similar report, would be extremely careful before committing on such an explosive story.

But the Imperial Household Agency, the tradition-steeped government body that runs royal affairs, was categorical in its denial.

"It is absolutely not true," Vice Grand Steward Shinichiro Yamamoto told reporters late Wednesday.

The emperor "has long refrained from discussing systematic issues out of consideration for his majesty's constitutional position," he told reporters.

The throne, which Japan claims to be one of the world's oldest, is held in deep respect by much of the public, despite being largely stripped of its mystique and quasi-divine status in the aftermath of World War II.

Akihito's father, Hirohito, in whose name Japan's military campaigns of the 20th century were prosecuted, was treated as a living god until defeat in 1945.

While the role of emperor is now largely ceremonial, it remains intensely important to right wingers, especially because of the monarch's position at the apex of the native Shinto religion.

Among their number is Prime Minister Shinzo Abe, who at the weekend scored an election win that may give him the momentum to soften Japan's pacifist constitution.

An abdication - for which there is no modern legal precedent - could throw a spanner in the works, tying up legislators and preventing any such constitutional backsliding.

Hidehiko Kasahara, professor of politics at Keio University, noted that Akihito has made clear that his ceremonial workload is getting harder to perform.

One of his sons in 2011 floated the idea of a retirement system for emperors after one of Akihito's illnesses, and a weekly magazine in 2013 carried a report similar to the current wave of speculation.

The emperor, who has suffered from numerous health issues, including prostate cancer and heart problems, himself spoke late last year of his growing limitations.

He acknowledged making "mistakes" in his duties, which range from native Shinto religious ceremonies to visiting residents in regions hit by Japan's frequent natural disasters.

Akihito may also be cognisant of the public awkwardness of his father's death from cancer in 1989.

Preceded by several months of serious illness, it led to a virtual paralysis of society as people became reluctant to engage in normal leisure activities lest being seen as insensitive to Hirohito's suffering.

Akihito's reign as emperor, now in its 28th year, has been characterised by an unprecedented openness, and some of his carefully crafted public comments have drawn attention.

He has touched on sensitive issues, including his own ancestry, suggesting in 2001 that some of his forebears came from the Korean peninsula.

While mainstream historians generally agree this is accurate, the suggestion was uncomfortable for some Japanese who like to think of their nation as ethnically homogeneous.

He has also made efforts to acknowledge suffering caused by Japan's 20th century warring in visits to the Philippines and other Pacific battle spots, and last year in Tokyo expressed "profound remorse" for the war.

Jeff Kingston, professor of Asian Studies at Temple University Japan, said he was not that surprised by the news given Akihito's health issues.

But lost in discussions, Kingston stressed, was not so much the technical issue of abdication but rather what Akihito has meant to his country as domestic "healer in chief" and "emissary of regional reconciliation" abroad.

"His acceptance of Japan's war responsibility and his apology diplomacy have done a great deal to raise Japan's stature in the comity of nations," he said.