A plan has emerged within the Japanese government to enable female members of the imperial family to keep their royal status even if they marry a commoner, sources with knowledge of the matter said Sunday, thus maintaining their branches of the family.

The current rules require female royals to give up their imperial status when they marry a commoner, causing the family to become progressively smaller.

File photo shows Princess Mako (R) and Kei Komuro attending a press conference in Tokyo on Sept. 3, 2017. (Kyodo)

The dwindling number of imperial family members has again become a focus ahead of the expected marriage later this year of Princess Mako, 29, a niece of Emperor Naruhito, to her boyfriend Kei Komuro.

The imperial household includes the emperor's family and four branches. Under the current patrilineal imperial succession rules, Princess Mako's brother Prince Hisahito, 15, is second in line to the Chrysanthemum Throne and the only heir of his generation.

The plan floated within the government intends to keep the branch numbers unchanged by retaining married female members or through the adoption of male heirs from former branches of the imperial family who abandoned their status in 1947, according to the sources.

That means Princess Aiko, 19, the only child of Emperor Naruhito, 61, and Princess Kako, 26, Princess Mako's sister and the other daughter of the emperor's brother Crown Prince Fumihito, 55, would likely remain in the imperial household even after marriage.

The plan also contemplates maintaining two other imperial family branches by allowing princesses to retain their status.

The aim of keeping the current branches is to create an environment in which the imperial family can support Prince Hisahito.

The government says it would seek to make sure the will of female members will be fully respected under the plan and carefully study its feasibility, the sources added.

"Unless we secure a certain number of imperial family members this way, we won't be able to have enough royals who can support Prince Hisahito," one government source said.

The branch led by Emperor Naruhito's uncle Prince Hitachi, 85, third in line to the throne, would explore the adoption of male heirs from the 11 now-abolished collateral branches who share with the imperial family a common ancestor some 600 years ago as the prince and his wife have no children.

Of the current 18 imperial family members, including former Emperor Akihito, 87, and former Empress Michiko, 86, who no longer perform official duties, 13 are women. With the expected departure of Princess Mako, the number of unmarried women would fall to five.

A Japanese government source said last week Princess Mako is expected to marry Komuro by year-end and may start a new life in the United States, where Komuro intends to pursue a legal career.

The plan to retain married female members is in line with discussions held by a government panel of experts to address the shrinking imperial household.

In July, the panel presented two options -- allowing female members who marry commoners to retain their imperial status and male heirs from former branches to be adopted into the imperial family by revising the 1947 Imperial House Law.

The panel also said in an interim report that its members agreed to maintain the current order of succession.

Even though opinion polls suggest overwhelming public support for allowing women or those descending from a female member of the imperial family to ascend the throne to secure a stable succession, conservative elements have been staunchly opposed to such an idea.

Reflecting conservative resistance, the plan excludes the possibility that female members could establish branches after marriage, which could pave the way for female monarchs or female-line emperors.

Government sources said last month the expert panel is considering not granting royal status to husbands and children of female members who would remain in the imperial household after marriage.

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