Japan's Emperor Akihito has strongly indicated he wants to step down, saying he fears his age will make it difficult to fulfill his duties.
The revered 82-year-old emperor's comments came in only his second-ever televised address to the public.
Emperor Akihito did not explicitly say he wanted to abdicate as he is barred from making political statements.
PM Shinzo Abe said the government would take the remarks "seriously" and discuss what could be done.
"Upon reflecting how he handles his official duty and so on, his age and the current situation of how he works, I do respect the heavy responsibility the emperor must be feeling and I believe we need to think hard about what we can do," he said.
Akihito, who has had heart surgery and was treated for prostate cancer, has been on the throne in Japan since the death of his father, Hirohito, in 1989.
In his 10-minute pre-recorded message, he said he had "started to reflect" on his years as emperor, and contemplate his position in the years to come.
Why can't the emperor abdicate?
Abdication is not mentioned under Japan's existing laws, so they would need to be changed for the emperor to be able to stand down. The changes would also have to be approved by parliament.
What do the public think?
Most support the emperor's desire to step down - a recent survey by the Kyodo news agency found more than 85% saying abdication should be legalised. But the move is opposed by some more conservative sections of Japanese society.
Is this the first time a revision of the law has been discussed?
A debate about whether or not a woman would be able to ascend the throne was triggered in 2006 when the emperor had no grandsons, but was postponed after a boy was born to the imperial family.
What does the emperor do?
The emperor has no political powers but has several official duties, such as greeting foreign dignitaries. Japan's monarchy is entwined in the Shinto religion and the emperor still performs religious ceremonies. He also plants and harvests a small rice paddy inside the palace while the empress raises silkworms.
If he were to abdicate, it would be the first time a Japanese emperor has stepped down since Emperor Kokaku in 1817.
A major milestone year marking the 70th anniversary of the end of World War II has passed, and in two years we will be welcoming the 30th year of Heisei.
As I am now more than 80 years old and there are times when I feel various constraints such as in my physical fitness, in the last few years I have started to reflect on my years as the Emperor, and contemplate on my role and my duties as the Emperor in the days to come.
As we are in the midst of a rapidly aging society, I would like to talk to you today about what would be a desirable role of the Emperor in a time when the Emperor, too, becomes advanced in age. While, being in the position of the Emperor, I must refrain from making any specific comments on the existing Imperial system, I would like to tell you what I, as an individual, have been thinking about.
Ever since my accession to the throne, I have carried out the acts of the Emperor in matters of state, and at the same time I have spent my days searching for and contemplating on what is the desirable role of the Emperor, who is designated to be the symbol of the State by the Constitution of Japan. As one who has inherited a long tradition, I have always felt a deep sense of responsibility to protect this tradition. At the same time, in a nation and in a world which are constantly changing, I have continued to think to this day about how the Japanese Imperial Family can put its traditions to good use in the present age and be an active and inherent part of society, responding to the expectations of the people.
It was some years ago, after my two surgeries that I began to feel a decline in my fitness level because of my advancing age, and I started to think about the pending future, how I should conduct myself should it become difficult for me to carry out my heavy duties in the way I have been doing, and what would be best for the country, for the people, and also for the Imperial Family members who will follow after me. I am already 80 years old, and fortunately I am now in good health. However, when I consider that my fitness level is gradually declining, I am worried that it may become difficult for me to carry out my duties as the symbol of the State with my whole being as I have done until now.
In coping with the aging of the Emperor, I think it is not possible to continue reducing perpetually the Emperor's acts in matters of state and his duties as the symbol of the State. A Regency may be established to act in the place of the Emperor when the Emperor cannot fulfill his duties for reasons such as he is not yet of age or he is seriously ill. Even in such cases, however, it does not change the fact that the Emperor continues to be the Emperor till the end of his life, even though he is unable to fully carry out his duties as the Emperor.
When the Emperor has ill health and his condition becomes serious, I am concerned that, as we have seen in the past, society comes to a standstill and people's lives are impacted in various ways. The practice in the Imperial Family has been that the death of the Emperor called for events of heavy mourning, continuing every day for two months, followed by funeral events which continue for one year. These various events occur simultaneously with events related to the new era, placing a very heavy strain on those involved in the events, in particular, the family left behind. It occurs to me from time to time to wonder whether it is possible to prevent such a situation.
As I said in the beginning, under the Constitution, the Emperor does not have powers related to government. Even under such circumstances, it is my hope that by thoroughly reflecting on our country's long history of emperors, the Imperial Family can continue to be with the people at all times and can work together with the people to build the future of our country, and that the duties of the Emperor as the symbol of the State can continue steadily without a break. With this earnest wish, I have decided to make my thoughts known.
I sincerely hope for your understanding.