Japan's Royal Family has relaxed its succession rules as it struggles with a lack of heirs - but the ban on female empresses remains.

Emperor Naruhito, 66, has just one daughter by his 62-year-old wife Empress Masako, Princess Aiko, 24, who was born in 2001 after the royal couple underwent fertility treatment.

Female births have far outweighed male births in recent generations of the family, and the dynasty currently has just three male heirs, one of whom is already in his 90s.

Under new legislation, passed last week, Japan's Imperial Family, the oldest continuous hereditary monarchy, can adopt distant male relatives over the age of 15 in a bid to address the concerns.

Female members will also be able to retain their titles and continue royal duties if they marry a commoner, a significant U-turn after Princess Mako renounced her titles to wed her college sweetheart, Kei Komuro, in October 2021.

Mako, the niece of Naruhito, 34, was forced, by law, to renounce her royal status and, following her marriage, moved to New York, where her husband is a lawyer.

The law forbidding women to ascend the throne remains, however, meaning Princess Aiko is still unable to become empress.

In addition to a lack of male heirs, many of the family are ageing - with seven members over 60, and only four members of the family being male.

As it stands, Emperor Naruhito's younger brother, Fumihito, 60, is the first in line to the throne, followed by Fumihito's son, Prince Hisahito, 19, and thirdly, the last eligible heir is the emperor's uncle, Prince Hitachi, who turned 90 in November.

Under the new bill, which will come into force after legal procedures, male descendants of 11 former imperial branches can be adopted into the family, despite being previously removed after the Second World War.

The new legislation marks the first amendment to the main text of the Imperial House Law since 1949, and is the biggest overhaul to the country's imperial system in decades, the BBC reported.

Emperor Naruhito's daughter Aiko lives a life moulded by the dynasty she's part of - surrounded by intense security and unable to have political views or vote.

The royal lives with her parents at The Imperial Palace in Edo Castle in Tokyo and works in the Youth and Volunteer Division for the Japanese Red Cross Society, a role she combines with her royal engagements.

For now, wedded to a life of public duty, Aiko will no longer need to look to the path her first cousin, Mako Komuro, formerly Princess Mako of Akishino, took; the former royal enjoys an upper-class quiet life in New York.

In 2017, Crown Prince Fumihito's eldest daughter sent shockwaves through the country when she announced she was giving up her royal title to marry Kei Komuro, her long-term boyfriend.

Mako and Komuro met at Tokyo's International Christian University in 2013 and became engaged in secret before announcing their intention to marry in September 2017, sparking huge scrutiny. 

Their wedding in October 2021 was distinctly low-key. It was met with protests and held behind closed doors without any of the pomp and pageantry of other Japanese royal weddings, which traditionally include a reception, banquet, and a parade through the streets of Tokyo.

Reading out a prepared statement, Mako defended her decision to marry, describing Kei as 'irreplaceable' and saying 'our marriage is a necessary step for us to be able to protect our hearts.'

Mako also surrendered a £1million payment she was entitled to, and became the first imperial family member since World War II to not receive the money.

Now living simply as Mako Komuro, the former princess lives in a luxurious but modest - at least according to the proportions of the Imperial Palace - apartment in the Big Apple, with the pair regularly pictured on shopping trips together in the city.

The Japanese imperial family is considered to be the world's oldest monarchy, with an unbroken line of male succession that can be traced back two millennia.

Mythology, recognised by the Imperial House, explains the legendary Emperor Jimmu, said to be a descendant of a sun goddess and a storm god, as the first of 126 Japanese emperors leading up to the current Emperor Naruhito.

European monarchies, meanwhile, have largely accepted female rulers, but a small, powerful group of traditionalists, many of whom are members of Japan's ruling Liberal Democratic Party, say that thousands of years of tradition would be broken should women be allowed to rule.