Japan's Parliament voted on Friday to enshrine male-only succession to the imperial throne, formally revising the Imperial House Law that has governed the world's oldest continuous monarchy. The decision effectively bars Princess Aiko, the only child of Emperor Naruhito and Empress Masako, from ever inheriting the throne, despite warnings from experts that restricting succession to men in the paternal line will accelerate the decline of an already shrinking and ageing imperial family.

The law being revised dates back to the 1800s, while the imperial line itself traces its origins back roughly 1,500 years. To offset the dwindling pool of eligible male heirs, the revisions will allow distant male relatives to be formally adopted into the imperial family, a measure intended to keep the succession line viable without opening it to female members.