Japan's parliament tweaked the imperial succession law on Friday but maintained the bar on women emperors -- despite surveys suggesting wide public support for the idea.
The future of the imperial household -- mythically descended from the Shinto sun goddess Amaterasu -- depends currently on Prince Hisahito, the 19-year-old nephew of Emperor Naruhito, 66.
If Hisahito -- a fan of dragonflies, who is currently studying biology and is not married -- has no son, then under the rules as they stand he will have no heir and the bloodline will end.
There have been eight female emperors on the Chrysanthemum Throne in Japan's imperial family, whose divine status was renounced after World War II.
But an 1889 imperial house law stipulated that only men could become emperor, and only through the paternal line. This was carried over in 1947 into the current Imperial Household Law.
This rules out the popular Princess Aiko, 24, daughter of Naruhito, or any other royal woman ever becoming emperor.
The bill, passed by the upper house on Friday, allows the adoption of male distant relatives aged over 15 back into the imperial family -- as long as they are single -- and for their future sons to become eligible to ascend the throne.
They are members of 11 families that left the imperial register after Japan's defeat in World War II.
The new rules also end the practice of women having to lose their royal status after marrying a commoner, although because of their gender their children still cannot become emperor.
The legislation passed after wrangling within the ruling Liberal Democratic Party (LDP) led by Japan's first woman prime minister, Sanae Takaichi, who opposes female succession.
Seiichiro Murakami, a veteran of the conservative LDP, said after the bill passed the lower house on July 10 that it was "utterly outrageous" to rule out Aiko becoming emperor.
Asahiro Kuni, 81, a member of one of the 11 imperial branches, has also said he would advise his grandchildren to refuse the opportunity of becoming royals.
"By the age of 15, a person has grown up breathing the air of freedom," Kuni told the Asahi Shimbun daily.
A poll conducted by the Mainichi Shimbun last month found only 23 percent of people in favor of the sons of re-adopted imperial family members becoming emperor, and 34 percent against.
By contrast, more than 70 percent supported a woman emperor, and 40 percent a matrilineal one.
An Asahi Shimbun poll in May also showed 72 percent of respondents in favor of changing the rules to allow women to ascend the throne.
The imperial family now has 16 members in total, including five men -- retired emperor Akihito, 92, his brother, 90, the emperor, his brother, and Hisahito.
Hideya Kawanishi, a professor at Nagoya University and expert in Japan's emperor system, told AFP that the new bill "fails to reflect public opinion".
"For the conservative camp of the ruling Liberal Democratic Party and the likes, the overriding goal is to preserve the male-line, male-only succession to the throne, and that is precisely why they do not want to listen to the voices of the people," he said.
"I believe these amendments carry the risk of undermining public support for the symbolic Emperor system."
"Maybe it would have been good if they talked more about the possibility of female emperors and other stuff reflecting modern society," office worker Yoshiki Yaguchi, 66, told AFP while walking in the Yurakcho area of Tokyo.
"The male chauvinism has to be scrapped," high-school student Yumi told AFP as she came out of a shopping mall with her like-minded friend, Misa, also 17.
