Emperor Akihito's role is purely ceremonial in Japanese society, but old age and ill-health now limits his work [EPA]

Prince Akishino, who is in second in line to the Japanese imperial throne, has said the country should look to setting a retirement age for its emperor, just days after his increasingly frail father was discharged from a lengthy hospital stay.

The younger son of the emperor was voicing a rare public opinion to reporters ahead of his own 46th birthday.

"I think it will become necessary," he said, when asked by a reporter to comment on an idea of setting a retirement age for Japanese emperors.

"When you pass a certain age, it gradually becomes difficult for people to do various things," said the prince, who called for "more discussion" on the subject.

The comments, published in the Japanese press on Wednesday, came after the prince's 77-year-old father, Emperor Akihito, resumed public duties following a 19-day stay in hospital, where he was treated for bronchial pneumonia and fever.

Japan's royal members rarely comment on public or political matters, including those touching on the affairs of their own cloistered family.

Akishino's remark comes as Japan is exploring ways to maintain the royal household in modern times.

Empress Michiko, also expressed concern last month about her husband's failing health, but said she stands beside the emperor while listening to the advice of physicians.

Akihito underwent surgery for prostate cancer in 2003 and still receives treatment.

The emperor did not attend a welcome ceremony this month for the visiting king and queen of Bhutan, the first time he has missed a meeting with a state guest since he ascended to the throne in 1989, following the death of his father Emperor Hirohito.