Traditionalists say the royals could yet have a boy

The changes would allow the newborn Princess Aiko to become second in line for the imperial throne.

Under current legislation, the royal family would face a succession crisis if no boys are born in the future.

Correspondents say most people in Japan are in favour of a female ruler. But they say the idea is an abomination to some traditionalists.

However, Princess Kikuko, writing in the monthly magazine Fujin-Koron, pointed out there were precedents in Japan's history to support the idea of a female monarch.

"I think it is possible that a female member of the imperial family could ascend the throne as the 127th reigning monarch, and that would not be unnatural, considering the long history of Japan," she wrote.

'Up to the stork'

Japan has had eight female monarchs in the past, most recently in the 18th century.

There were celebrations across Japan at the birth

Princess Aiko was born last month to Crown Prince Naruhito - the current heir to the throne - and his wife Crown Princess Masako, after eight years of marriage. The line of succession could theoretically run out unless they have a son.

Princess Kikuko said it was "up to the stork" whether 38-year-old Princess Masako would give birth to a boy in the future.

And she argued Japan could change its succession laws, citing female British monarchs as successful role models.

"Like the Elizabethan and Victorian eras in Britain, there were many examples in foreign countries where a nation thrived under the rule of a queen," she wrote.

It is highly unusual for a member of the royal family member to write for a popular magazine.

According to myth, the royal family is directly descended from the sun goddess.

Most historians agree the family is at least 1,500 years old.