In an unprecedented expression of public dissent, Prince Tomohito of Mikasa, the cousin of Emperor Akihito, claims that the proposal violates millennia of tradition and threatens the long-term survival of the Japanese monarchy.

His remarks, in a recent article, have caused dismay in the imperial family because they breach the ban on its members becoming publicly embroiled in politics or controversy.

The article, published in a charity newsletter, invokes Emperor Jimmu, the mythical first emperor said to have been descended from the Sun Goddess, in justifying the importance of imperial succession through the male line.

“For 125 generations, from the first Emperor Jimmu in the era of myth until now, without a single exception, the succession has been restricted to the male line only — this fact makes it something precious,” Prince Tomohito wrote. He added that if the system were changed, some day people would argue that the emperor was no longer needed.

His remarks come as Japanese conservatives intensify a campaign against the proposal to allow a reigning empress. A month ago a group of scholars proposed the solution supported by Prince Tomohito — that members of aristocratic families that were stripped of their titles after the Second World War be restored and adopted into the imperial family. This month a member of one of the former imperial houses, the Takeda family, will publish a book making the same argument.

The succession crisis is caused by a simple fact: since the birth of the Emperor’s younger son, Akishino, almost 40 years ago, the imperial family has had no sons. Crown Princess Masako, 41, underwent fertility treatment in the hope of producing an heir and gave birth to her only child, Princess Aiko, in December 2001. But under Japan’s Imperial Household Law, only a male child descended from an Emperor can succeed.

An advisory panel, appointed by Junichiro Koizumi, the Prime Minister, will deliver a report in the next few weeks recommending a change in the law to permit a reigning empress.

The Emperor has been careful not to express any view on female succession publicly, but there is strong support for the proposal within the Imperial Household Agency, the highly conservative bureaucracy that regulates the lives of the monarch and his family. Opinion polls show that 80 per cent of the public want to see an Empress Aiko.

Eight women have reigned as Empress, the last being Go-Sakuramachi, between 1762 and 1771. But all yielded to the male line after death or abdication.

Three times in the past — in the 6th, 15th and 18th centuries — distant relatives from collateral branches of the imperial family ascended to the throne after the main line ran out of male heirs.

Eleven noble families lost their imperial status in 1947 under reforms introduced by the US. Seven of them survive, and five of them have unmarried sons of marriageable age. The eight eligible bachelors include Asatoshi Kuni, 33, who works for a trading house, and Tsuneyasu Takeda, 29, a businessman and writer, whose book, Untold Story of the Imperial Family, will be published this month.

There may be a certain amount of self-interest on the part of those arguing for this change. Prince Tomohito has two unmarried daughters who would be prime candidates to marry newly restored imperial princes in the hope of creating a male heir. He would then become the grandfather of the future emperor.

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