The saga has been followed by Japan's gossipy tabloid magazines, which have written of strife between Naruhito and Akishino over whether public duties take precedence over personal fulfilment and compared Masako, sometimes unfavourably, to the demure Kiko, who seems satisfied with a role as wife and mother.

Experts agree reform of the succession law will be needed eventually, despite the birth of the boy, since ensuring male heirs is difficult without a royal concubine.

The practice of emperors taking concubines ended when Emperor Akihito's late father, Hirohito, refused to take one.

Conservatives, however, would prefer to revive princely houses abolished after World War Two to expand the pool of possible male heirs, rather than let women reign.

"I think it solves the short-term problem but they still have got a major issue on their hands," said Kenneth Ruoff, a professor at Portland State University and author of "The People's Emperor".

"They have no reserve in terms of if something should happen to the child and down the road, they will have a crown prince and emperor with no other princes around."

Japan has had eight reigning empresses, the last in the 18th century, but conservatives stress they were stop-gap rulers.

Share prices of baby goods makers climbed on Monday on hopes the royal birth could help lift the nation's slumping birth rate from its record low, but sociologists rejected notion that women would start having more babies just because of the royal birth.

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