And then there were three - Japan's shrinking pool of imperial heirs

Surveys show a majority of Japanese favour letting women take the throne, and then passing it on to their children, but conservatives disagree, and they are key to Prime Minister Shinzo Abe's support.

In 2006, the then-prime minister, Junichiro Koizumi, was close to proposing a revision of the males-only rule, but that was shelved with the birth of Prince Hisahito, ending a 41-year drought in male heirs.

Historically, Japan's imperial line was preserved by a combination of concubines and cadet royal families called "miyake", who could supply a male heir in a pinch.

Japan had several female emperors in the premodern period but that practice ended with the enactment of the Imperial House Law in 1889, which stipulated males-only succession.

Emperor Meiji, who died in 1912, was the last monarch to have concubines, and cadet families were stripped of royal status by the Allied Occupation after Japan's World War Two defeat.

When parliament in 2017 enacted a special law to allow Akihito to abdicate - itself a controversial move for conservatives - lawmakers also adopted a non-binding resolution asking the government to consider how to ensure stable succession.

One change could be to let female royals remain in the imperial family after marrying commoners, so they could at least share imperial duties. But conservatives are cautious about that too as they see it as a first step towards female succession.