The Japanese parliament has approved a bill to relax imperial succession rules, amid concerns over the dwindling size of the imperial family.

The bill, passed by the upper house on Friday, allows the imperial family to adopt distant male relatives over the age of 15 and lets women keep their royal status after marrying outside the family.

But it does not change the law barring women from ascending the throne despite wide public support for a female emperor, meaning Princess Aiko, the only child of the current emperor, is still not eligible to succeed the throne.

The bill cleared the lower house last week, and will move through the final legal procedures before the changes take effect.

Japan has the world’s oldest continuous hereditary monarchy, with a lineage that’s believed to span more than 2,600 years.

Currently first in line to the throne is 60-year-old Fumihito, the emperor’s younger brother.

Fumihito’s son, 19-year-old Prince Hisahito, is second in line.

Third-in-line, and the last eligible candidate for the throne, is the emperor’s 90-year-old uncle.

Without any amendments to the law, the line of succession will end if Prince Hisahito does not have a male child.

However, with the new bill, male descendants of 11 former imperial branches could be adopted back into the family. These family branches had been removed following the Second World War.