Japanese Princess Mako, the eldest granddaughter of Emperor Akihito, will wed a former college classmate, Japanese media reported, heating up debate on the ever-shrinking royal family since she must become a commoner after marriage.

Japan's cabinet is expected to approve a bill tomorrow to allow Akihito, 83, to step down, the first abdication by a Japanese emperor in nearly two centuries, after he said last August he feared age would make it hard to fulfil his duties.

But the legislation will make no reference to the controversial topics of whether to revise a males-only succession law or to allow women to stay in the imperial family after marriage, a move conservatives fear would be a first step to letting females inherit the throne.

Asked about the problem of the shortage in royals, Chief Cabinet Secretary Yoshihide Suga told a news conference on Wednesday: "There is no change in our view to proceed with consideration of steps to ensure stable imperial succession."

The 25-year-old Mako's unofficial fiancé, Kei Komuro, appeared before media cameras on Wednesday outside the Tokyo law office where he works, a day after the news of their engagement broke. The Imperial Household Agency declined to comment.

The shrinking royal population – which mirrors the broader trend of Japanese society – has raised concerns that the youngest prince may also be the last.

"Now is not the time for me to comment, but I want to speak at the right time," Komuro, 25, repeatedly told reporters.

Japanese Emperor Akihito and Empress Michiko smile with their family members. Photo: Imperial Household Agency of Japan via Reuters

There are only four heirs to the throne: Akihito's two middle-aged sons, whose wives are in their early 50s, Akihito's octogenarian brother, and Prince Hisahito, the 10-year-old son of Akihito's younger son.

Akihito has only four grandchildren, the other three of whom are female: Mako, her younger sister, Kako, and Crown Prince Naruhito's daughter, Aiko. The shrinking royal population – which mirrors the broader trend of Japanese society – has raised concerns that the youngest prince may also be the last.

"Under the present system, there is the risk that Hisahito will be the only one left in the imperial family," Keio University Professor Hidehiko Kasahara was quoted by media as telling an experts panel that studied the abdication issue. (Reuters)