TOKYO (Reuters) - The 100-year-old uncle of Japanese Emperor Akihito, Prince Mikasa, died on Thursday, leaving only four heirs to the Chrysanthemum throne, the Imperial Household Agency said.

Mikasa’s death coincides with renewed attention to the future of the aging and shrinking imperial family and whether women should be allowed to inherit the throne, breaking a males-only succession tradition that conservatives say is central to an imperial tradition stretching back 2,600 years.

Mikasa was the youngest brother of the current emperor’s father, Hirohito, in whose name Japan fought World War Two

The prince, a scholar of ancient Oriental history, taught at colleges, and served as honorary president of the Middle Eastern Culture Center in Japan and the Japan-Turkey Society.

Emperor Akihito, 82, hinted in August that he wanted to abdicate - a step unprecedented in modern Japan and not possible under current law. The remaining four male heirs include 10-year-old Prince Hisahito, the emperor’s only grandson.

The three older heirs are Akihito’s 80-year-old brother and his two middle-aged sons including Crown Prince Naruhito.</s>Prince Mikasa, uncle of Emperor Akihito, died on Thursday, government sources said. He was 100.

He has been in and out of the hospital due to pneumonia since May. He died at 8:34 a.m. on Thursday in a Tokyo hospital after his condition deteriorated, the sources said.

He was fifth in line to succeed the Chrysanthemum Throne. With his death, there are now four heirs in the Imperial family — Crown Prince Naruhito, Prince Akishino, Prince Hisahito and Prince Hitachi, the younger brother of Emperor Akihito.

The prince was the oldest member of the Imperial family, living through the reign of three different Emperors, in the Taisho, Showa and Heisei eras. He was the only living royal family member to have served in the military during the Sino-Japanese war from 1937 and the Pacific Theater of World War II.

Prince Mikasa was born on Dec. 2, 1915, the fourth son of Emperor Taisho. His eldest brother was Emperor Hirohito, posthumously called Emperor Showa.

The prince served as an officer in the Imperial Japanese Army after graduating from the army’s college. He was posted to Nanjing, China, under a pseudonym for one year from 1943. In a book published after the end of the war, he expressed a sense of guilt over his failure to fully grasp the criminality of war.

With his wife, Princess Yuriko, Prince Mikasa had three sons and two daughters. Prince Takamado, Tomohito and Katsura all preceded him in death, in 2002, 2012 and 2014, respectively.

The prince was known as a scholar of ancient Oriental history and served as honorary president of the Middle Eastern Culture Center in Japan and the Japan-Turkey Society.</s>The death of Prince Mikasa — the 100-year-old uncle of Emperor Akihito who was one of only five men in line for the Chrysanthemum throne — has once again put the spotlight on the issue of Imperial succession and the dwindling size of the Imperial family.

After the prince’s death Thursday, Imperial Household Agency Grand Steward Shinichiro Yamamoto acknowledged at a regular news conference that the succession issue is a problem and something that poses “a major challenge.”

Behind the scenes, the agency has called on the government to debate ways to secure a stable Imperial succession protocol, including the possible enthronement of female members, accession by heirs of female lineage, or the establishment of female branches of the Imperial family.

Prime Minister Shinzo Abe’s government, however, has not asked a government panel studying the Emperor’s possible abdication to deal with such topics due to their lower priority and fears of how long the discussions will last.

The prince’s death leaves only four eligible heirs to the throne. They are, in order, Crown Prince Naruhito, 56; his younger brother, Prince Akishino, 50; Prince Hisahito, 10; and the Emperor’s 80-year-old brother, Prince Hitachi. Only male members of the family line can ascend the throne under the 1947 Imperial House Law.

In the past 15 years, the Imperial family has had just two births — Princess Aiko, the 14-year-old daughter of Crown Prince Naruhito and Princess Masako, and Prince Hisahito, who is Prince Akishino’s son. Seven members have left the family due to death or marriage, bringing the total to 19.

The panel was established last month to discuss how to alleviate the burden on the 82-year-old Emperor following his rare video message in August, in which he expressed his readiness to abdicate should his health deteriorate in the future.

At present, only posthumous succession is allowed as the Imperial House Law lacks a provision for abdication.

More than a decade ago, there was intense debate under the government of former Prime Minister Junichiro Koizumi about the possibility of revising the Imperial House Law to enable female members or heirs of female lineage to ascend the throne.

Koizumi eventually gave up on submitting a bill on the matter in September 2006, a move that many believe was linked to the birth of Prince Hisahito — the first new male member in 41 years — the same month.

Abe, who was chief Cabinet secretary at the time, played a key role in the decision, according to sources close to the matter.

Koizumi was concerned, despite the birth of Prince Hisahito, that Imperial succession may become unsustainable in the future, the sources said.

Abe, for his part, pushed for dropping the idea of seeking an amendment, saying, “Now is not time to revise the law,” the sources added.

“The idea of maintaining the Imperial throne only by heirs to male lineage is close to (Abe’s) strong belief,” said a source from the Prime Minister’s Office.

Abe is known to be against extending eligibility for the throne to Imperial family members of female lineage and enabling female members to stay within the family after marriage.

Abe said in a contributed article to monthly publication Bungei Shunju’s February 2012 edition that he was “clearly opposed” to having a female emperor.

During a Diet session this past February, Abe said it will be important to consider “the weight of the fact that succession by heirs of male lineage has continued without exception since ancient times.”

In 2012, the government led by the former Democratic Party of Japan, the opposition predecessor of the Democratic Party, studied the idea of establishing female branches of the Imperial family so women could stay within the family after marriage.

However, their research drew opposition from the conservative electorate.

The debates pertaining to those issues died down as Abe and the Liberal Democratic Party returned to power later in the year, with Abe expressing a negative stance to such ideas.

During a news conference on Thursday morning immediately after Prince Mikasa passed away, Chief Cabinet Secretary Yoshihide Suga ruled out the possibility of adding the issue of how to grapple with the decreasing number of Imperial family members to the topics to be discussed by the government panel.

Abe’s government is seeking to avoid long debate as it aims to submit a relevant bill to an ordinary Diet session next year to make it possible for only Emperor Akihito to abdicate, according to a ranking government official.

Later Thursday, Suga stressed the importance of measures dealing with the aging and shrinking of the Imperial family, saying at an afternoon news conference they are “problems that should not be unnecessarily shelved,” but he did not elaborate or clarify when the discussions would start.

The issue of sustainability of Imperial succession was not touched on in the panel’s second session Thursday, despite the fact it was held hours after the prince’s death, according to a government official who took part.

A former senior government official who took part in discussions over the revision of the Imperial House Law said, “The crisis of the Imperial household will worsen if the Abe administration doesn’t discuss the matter.”</s>Emperor Akihito’s uncle Prince Mikasa, who died of heart failure Thursday at the age of 100, spent the last years of his life peacefully at his residence in Tokyo, taking strolls and enjoying his daily studies and exercise.

The prince, before he became wheelchair-bound in recent years, often took walks with his wife, Princess Yuriko, 93, in the Akasaka Imperial Grounds and enjoyed sunbathing in his garden on sunny days, according to palace staff and others who knew him.

He would wake up at 4 a.m. every morning, listen to a foreign-language learning program on the radio and exercise for about 30 minutes.

An avid reader of newspapers, Prince Mikasa also enjoyed watching sumo wrestling and popular music shows on TV. He abstained from eating meat.

Palace staff and others say the prince, who appeared hale, was always supporting Princess Yuriko, who needs a stick to walk.

Prince Mikasa would leave his residence about once a week to attend birthday ceremonies for Imperial family members, or to have his hair cut.

As a researcher, the prince pursued his academic interests and devoted himself to the study of ancient Oriental history at the Faculty of Letters of the University of Tokyo.

Later, he taught at such universities as Tokyo Woman’s Christian University and the Tokyo University of the Arts.

The prince made many appearances on radio and television cultural programs and was affectionately called the “Imperial scholar.”

He survived his three sons, Princes Takamado, Tomohito and Katsura, who died in 2002, 2012 and 2014, respectively.

When his granddaughter Princess Noriko, the second daughter of Prince Takamado, married in October 2014, Prince Mikasa attended her wedding party in Tokyo..

During his hospitalization in an intensive-care unit at St. Luke’s International Hospital, Princess Yuriko frequently visited him along with other Imperial family members.

Although his health was up and down during his hospitalization, the prince responded to visitors, according to a person familiar with his condition. In June, the Emperor and Empress Michiko also visited him there.

The Imperial Household Agency said it will consider the date and venue of the funeral for the prince, adding that the Imperial family will go into mourning for seven to 90 days beginning Thursday.

The agency also said this year’s autumn garden party hosted by the Emperor and Empress will be canceled.
Japan's Prince Mikasa, the oldest member of the Imperial Family, dies at the age of 100.