TOKYO - Japan will need to hold "prudent and thorough" discussions on the future of its royal lineage, Prime Minister Shinzo Abe said on Wednesday (Nov 22), as the world's oldest continuous hereditary monarchy is threatened by a lack of male heirs.

Women are barred from taking the throne, and Mr Abe's comments to Parliament came as the nation edges closer to deciding when its revered Emperor Akihito, who turns 84 next month, can abdicate.

Domestic media, citing informed sources, have floated two dates - March 31, 2019 and April 30, 2019. The latter is said to be favoured by the government, as March is traditionally busier given that it marks the close of a fiscal year.

Mr Abe will chair a meeting of the 10-member Imperial Household Council to discuss the date of Japan's first abdication in 200 years next Friday (Dec 1). The panel comprises civil servants and officials from the Imperial Household Agency that manages royal family matters.

"Maintaining the stability of imperial succession is an extremely crucial issue that relates to the foundation of Japan as a country," Mr Abe said.

"We will consider our way forward on the gravity of such factors as the continuity of Japan's imperial bloodline, which has been maintained without exception since ancient times."

Crown Prince Naruhito, 57, will ascend the throne the day after his father steps down, marking the start of a new era. Thereafter, there will only be two male heirs to the throne in a bloodline that dates back to 660BC: Prince Akishino, 51, the crown prince's younger brother; and Prince Hisahito, 11, who is Prince Akishino's only son.

Talks were held over a decade ago on the possibility of female monarchs, but these were quickly shelved with Hisahito's birth and have never been resuscitated.

Currently daughters born into the royal family have to give up their status and become "commoners" when they get married.

This will be the case when Princess Mako - Emperor Akihito's eldest granddaughter - marries her university sweetheart, Kei Komuro, next year. Both are 26.

Their wedding will be held on November 4 at Tokyo's Imperial Hotel, domestic media, citing Imperial Household Agency sources, reported on Wednesday (Nov 22).

They will officially get engaged in a ceremony on March 4, which will kick off a series of traditional bethrothal rites leading up to their marriage.

After Princess Mako's departure from the imperial family, the number of royal members will drop to 18, of whom 13 are women and eight are below the age of 40.

The question of a male heir was not an issue in pre-war Japan, when the imperial family comprised as many as 12 "collateral branches". The post-war 1947 Constitution drafted by the US not only stripped the emperor of his status as a "divine ruler", but also the royal status of the other 11 branches as a cost-saving step.

The shrinking pool of royals to shoulder imperial duties has forced Tokyo to consider the idea of forming "female-led imperial branches", which will enable Princess Mako to continue her royal duties after marriage.

But the conservative Liberal Democratic Party (LDP) government and its affiliated ultra-nationalist Nippon Kaigi lobby group appear to be resistant to the idea of allowing sons born to a princess with a "commoner" husband to take the throne.

In August last year, Emperor Akihito, who has had heart surgery and treatment for prostate cancer, stunned Japan with a rare nationally-televised address when he hinted at his desire to step down from the throne.

He cited old age as having impeded him from fully executing his duties, despite the fact that the Imperial Household Agency had in recent years sought to lighten his workload.

The emperor has no political power in Japan, occupying a ceremonial role defined in the Constitution as a "symbol of the state and unity of the people".

Emperor Akihito's reign, which began in 1989, has been named the Heisei era, which means achieving peace.

In many ways, he appears to hold a divergent view from the apologist stance towards World War II taken by the government and Nippon Kaigi. His reign has been marked by active efforts to soothe the wounds of a war fought in the name of his father and predecessor, Hirohito.

He has also travelled extensively around Japan to uplift citizens in disaster-stricken areas.