A senior palace source told The Times that concern about the lack of male heirs has forced the Japanese court to begin studying the possibility that Crown Prince Naruhito may be succeeded by one-year old Princess Aiko, his only child.

She would be the first Empress in two and a half centuries to reign over the world’s oldest hereditary monarchy. Such a change would represent the defining moment of the reign of the present Emperor, Akihito, who during his 15 years on the throne has discreetly modernised the imperial institution.

But Japanese courtiers are concerned that debating such a fundamental change publicly will provoke questions about the very existence of the 1,500-year-old monarchy.

Senior officials of the Imperial Household Agency, the equivalent of the Buckingham Palace bureaucracy, have always denied that any changes to the Emperor system are being considered.

But, privately, senior sources in the court admit that planning has begun and that unless the Crown Prince, who is 43, and his wife, Princess Masako, 39, have a son in the next few years, a public debate will begin formally.

The Prince and Princess tried to have a baby for eight years. Princess Masako suffered a miscarriage before conceiving again in 2001 after she was treated by one of Japan’s leading fertility specialists. The couple’s delight in their daughter has been obvious during their public appearances. But the fact remains that no boy has been born into the family for 37 years and that under the Imperial Household Law, women are barred from ascending to the throne.

“This is a politically sensitive issue, so we can’t publicly admit we are researching the possibility,” a senior palace source said. “But as a matter of fact we are, and we would be negligent not to.”

Seven years ago an imperial envoy was sent on a study tour to the Netherlands, Belgium and Denmark, where succession laws have been changed to allow for rule by a queen.

The possibility of a reigning empress was debated in Japan during the formulation of the country’s first modern constitution in 1889 and immediately after the Second World War.

The danger of raising the issue now is that it will provoke confrontations between extreme right-wingers, who oppose any changes in imperial tradition, and left-wingers, who regard the monarchy as a symbol of wartime militarism.

After the Second World War, Emperor Akihito’s father, the late Hirohito, narrowly escaped being tried as a war criminal. The abolition of the monarchy was considered during the US occupation. “The last thing we want is for public opinion to be divided over this,” the palace source said. “Until recently we had political parties which advocated abolishing the system.”

Emperor Akihito, 69, recently underwent surgery for prostate cancer. Although he is said to have made a complete recovery, his illness has added urgency to the question of the succession. Since ascending to the throne in 1989, he has done much to win back broad support for the monarchy, and a change in the succession law would be in keeping with his modernising approach. It would also represent a symbolic boost for Japanese women, who remain poorly represented in politics, academia and the top echelons of business.

Royal heritage

• 1868: Emperor Meiji’s accession to Chrysanthemum Throne restores authority on state affairs to the Japanese Emperors, ending the domination of the shoguns

• 1912: Death of Meiji, who is succeeded by Emperor Taisho

• 1926: Emperor Hirohito accedes to the throne

• 1989: Emperor Akihito replaces Hirohito, who has died of cancer

• 2002: Emperor Akihito has cancer diagnosed on Christmas Eve

• 2003: Akihito undergoes surgery, which was reportedly a success

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