A member of the Japanese royal family has said the country should consider other options before allowing a woman to succeed to the throne.
Crown Princess Masako has failed to produce a male heir
Prince Tomohito, a cousin of Emperor Akihito, said Japan should expand its royal circle and reintroduce concubines to increase the chances of a male heir.
His remarks come as a panel of experts considers the possibility of allowing women to take the throne.
This is because the two sons of the current emperor have no male children.
Some conservatives have opposed changing the succession laws, but the idea has wide support among the country's public.
Prince Tomohito's arguments were published in a private newsletter.
"The reason why the imperial family line is so precious... is due to the very fact that it's been, without exception, a male line," he wrote.
Currently only males can ascend the Japanese throne
Emperor Akihito has two sons, Naruhito and Akishino
If Naruhito died without a male heir, his brother, Akishino, succeeds
But he has no sons either
Their sister, Princess Sayako, is marrying a commoner so her children cannot ascend throne
"The question is whether it is a right thing to change the unique tradition and history so easily."
He said one option would be to reinstate former members of the old aristocracy who left the Imperial Family after Japan's defeat in World War II.
"Using concubines, like we used to, is also an option," he said. "I'm all for it, but this might be a little difficult considering the social climate in and outside the country."
A 10-member government panel has been debating the issue of female succession since January, and said last week that it would recommend that the rules be changed.
Japan's 71-year-old ruling emperor, Akihito, has two sons and a daughter, but none of them have so far produced a male heir to ensure the succession.
The Imperial Household's main succession hopes lie with Akihito's eldest son Crown Prince Naruhito and his wife Masako.
But analysts believe the pressure on Masako to bear a male heir contributed to stress-related illnesses which stopped her fulfilling official duties for more than a year.
If the rules were changed, her three-year-old daughter, Princess Aiko, could succeed instead.
Japan has had female monarchs before - between the 6th and 18th centuries - but all have reigned in emergency circumstances and none had children who then ascended the throne.