Japanese Emperor Akihito is 82, has health challenges and is laying the groundwork to relinquish his role. But he may not be allowed to, and even bringing up the subject may be unconstitutional. (Toru Yamanaka/AFP/Getty Images)

For 28 years, Emperor Akihito has been a steady and reassuring presence in Japan, a fact that many people here are reminded of on a daily basis. After all, 2016 is officially known as “Heisei 28,” marking Akihito’s time on the Chrysanthemum Throne.

But now, the 82-year-old “emperor for life” is laying the groundwork to relinquish his role and pass it on to his oldest son, Naruhito. That will be tricky. Not only is there no legal provision for him to abdicate, but even raising the prospect could be unconstitutional.

“Under the current law, he can’t abdicate, even if he wants to. There is no option but to carry on,” said Yasushi Kuno, a veteran journalist who for years covered the imperial family for the Nippon television network.

Akihito is scheduled to make a pre-recorded video statement to the Japanese people Monday afternoon, during which he probably will say that he is having difficulty carrying out his official duties.

He has had health issues — prostate cancer and heart problems — and, marking his birthday in December, he said there had been times when he had felt his age.

Emperor Akihito hinted that he wants to abdicate, but the move would be unprecedented under Japan's Imperial House Law, which says an emperor serves until death. (Jenny Starrs/The Washington Post)

“Even if he tries really hard, he can’t deny that his body is deteriorating,” which means he can no longer carry out all his official duties, Kuno said.

Surviving through samurais and shoguns and wars, an unbroken male line of emperors has endured in Japan for almost 3,000 years. They are said to be direct descendants of Amaterasu, the Shinto goddess of the sun.

Abdication was relatively common until 1817, when Kokaku became the last emperor to resign his post.

But the imperial system underwent a huge upheaval at the end of World War II, when the U.S. occupying forces allowed Hirohito, the current emperor’s father, to remain in his position but stripped him of his powers.

The emperor was reduced to being a ceremonial figurehead who would serve as a “symbol of the state and of the unity of the people,” according to the U.S.-written constitution. As such, he does “not have powers related to government,” meaning that he cannot say anything even remotely political.

That will cause some issues for Akihito, the only emperor to have begun his reign under the postwar constitution. Because there is no provision in the Imperial House Law for him to abdicate, even raising the idea would be considered political because it would require a parliamentary amendment.

“So he will be ambiguous, unclear,” said Takeshi Hara, a professor of politics who has written several books on the imperial system. “I think he will just express his feelings.”

Signs of the emperor’s wish to step down emerged last month when NHK, the public broadcaster, which has close ties to Prime Minister Shinzo Abe’s government, reported that it was under discussion.

The public has been supportive of the idea, with polls showing that between 77 percent and 90 percent of respondents say the government should create a system to allow the emperor to abdicate.

“If he feels old and tired, it’s okay for him to retire,” said Yukiko Sakurai, one of a group of four gray-haired women sitting in a Tokyo cafe last week. “He’s old. Maybe they should set an age limit on being emperor?”

Hirohito died at age 87; Akihito was 55 when he succeeded his father. His oldest son, Naruhito, is 56.

The Japanese public has warm feelings toward the current emperor. His father was considered to be “above the clouds,” so revered that Japanese people weren’t even allowed to look straight at him during the war.

“But the current emperor has a different style and talks directly to the people,” said Kuno, the journalist. This was particularly evident after the March 2011 earthquake and tsunami, when Akihito for the first time recorded a video message to the Japanese people, and then visited the disaster zone.

Any legal changes will take time, probably years, to usher through. But in the meantime, the emperor’s intentions probably will create headaches for Abe, whose top — and controversial — priority is revising the constitution to loosen the pacifism imposed on Japan after the war.

Abe’s government last month succeeded in winning the two-thirds majority needed in the upper house to try to make changes to the constitution.

The emperor has obliquely signaled that he disagrees with attempts to revise the constitution and has made efforts to atone for Japan’s wartime brutality.

“I hear the emperor feels a sense of crisis over the current political situation,” said Jiro Yamaguchi, a political scientist at Hosei University. “Abe’s position on constitutional revision is completely different from the emperor’s position of protecting the constitution.”

Akihito’s coming statement, he said, could trigger a drive among the public to keep the constitution as it is.

Talk of legal changes could put the brakes on efforts to revise the constitution, said Mari Miura, a political scientist at Sophia University. But it also could inject momentum into the efforts. “This could give a push to those on the revision side if all the changes could be reviewed together,” she said.

Abe has other reasons to be resistant to change in the royal status. He lobbied against efforts a decade ago, when the emperor had only granddaughters, to allow women to inherit the title.

Thorny legal questions aside, there are lots of logistical considerations, much like the Vatican had to grapple with when Pope Benedict XVI wanted to step aside. Where would Akihito live? What would he be called? “Retired emperor?”

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Yuki Oda contributed to this report.