Japan's parliament has passed a law allowing Emperor Akihito to become the first monarch to abdicate in 200 years.

Key points: Monarchy has shrinking royal population and few male successors.

Monarchy has shrinking royal population and few male successors. Accession of Crown Prince Naruhito likely at end of 2018.

Accession of Crown Prince Naruhito likely at end of 2018. Laws cleared both upper and lower houses in parliament.

The 83-year-old emperor expressed his apparent wish to abdicate last August, citing old age and health.

Under the law enacted on Friday, an abdication must take place within three years, but parliament put off a debate over how to tackle the shrinking royal population, including male successors.

The law clears the way for the first abdication by a Japanese monarch in almost two centuries and the accession of his 57-year-old son, Crown Prince Naruhito, probably late next year.

Family throne up in the heir

Emperor Akihito's abdication has rekindled concerns about a shortage of heirs.

Japanese Princess Mako (left), the eldest granddaughter of Emperor Akihito, will leave the ever-shrinking royal family by marrying a commoner. Princess Kako is pictured on the right. ( Reuters: file )

The current succession rules allow only men from the paternal bloodline and prohibit women from ascending the 2,000-year-old Chrysanthemum Throne.

Only women are forced to renounce their royal status when they marry a commoner, heating up debate for members of the royal family such as Princess Mako.

But the Prince's only child is a girl, and his younger brother Prince Akishino has two adult daughters and a 10-year-old son, Hisahito.

This means only one of the emperor's four grandchildren is an eligible heir.

After Naruhito's daughter was born, a government panel discussed the possibility of allowing female emperors, but the talk faded as soon as Hisahito was born.

The emperor, who has had heart surgery and treatment for prostate cancer, said in rare public remarks last year he feared age might make it hard for him to continue to fulfil his duties.

The soft-spoken emperor, the first Japanese emperor who was never considered divine, has worked for decades at home and abroad to soothe the wounds of World War II, fought in his father Hirohito's name.

Japan's Crown Prince Naruhito (left) is expected to take the throne in late 2018 with his wife Princess Masako (right). ( Facebook: I'm Proud to be Tongan )

Government support for emperor's new groove

Prime Minister Shinzo Abe's ultra-conservative government supported the current male-only succession.

In a vote televised live on local television, the upper house of parliament passed the bill with a handful of lawmakers sitting out the vote.

Japan's Prime Minister Shinzo Abe and the parliament passed laws for the emperor to abdicate for the first time in 200 years. ( AP: Gregorio Borgia )

It cleared the more powerful lower house last week.

The government will have to hammer out the details of the abdication, including timing, but local media reports have said it is likely to take place at the end of 2018, which would mark three decades on the throne for the emperor.

Mr Abe's government avoided taking up divisive issues involving the status of female royals, which would require a time-consuming and broader overhaul of the outdated 1947 Imperial House Law.

His ruling party reluctantly agreed to adopt a non-binding attachment to the law calling for the government to study ways to improve the status of princesses, including allowing them to keep their titles so they can make up for the declining royal membership and perform some public duties.

The last emperor to abdicate was Kokaku in 1817.

AP/Reuters