The leak so early in Princess Kiko’s pregnancy appeared designed to scupper plans by Junichiro Koizumi, the Prime Minister, to change imperial law so that Aiko, Crown Prince Naruhito’s four-year-old daughter, can succeed to the throne in the absence of a male heir.

Japan must now endure months of suspense before it learns whether Princess Kiko’s baby is a boy, which would end the succession crisis.

If it is a girl, however, the battle to allow Aiko to become Japan’s first empress since the 18th century would resume without its champion, as Mr Koizumi said that he would step down in September.

The leak caused astonishment across Japan, and there was speculation over its timely appearance. Many believe that word of Princess Kiko’s pregnancy may have come from the staff of her husband, Prince Akishino. Its effect, say analysts, will be to quash all talk of imperial law revisions until after the baby is born.

But the media reported that the leak, to NHK, the state broadcaster, may have occurred before Princess Kiko had a chance to telephone her husband with confirmation of their news. That would suggest it came from the hospital in Tokyo where the Princess had an ultrasound scan yesterday.

Either way, the leak plays perfectly to the swelling and vocal group of politicians and academics who fiercely reject the Prime Minister’s solution to the succession crisis.

The battle has been looming for several years and the pressure on Princess Kiko, 39, and Prince Akishino, 40, will be enormous. For Prince Akishino, becoming the father of a future emperor would demand a sharp increase in royal duties and possible shift in demeanour. The alleged romances and partying of his younger days remain a favorite subject of gossip magazines.

The couple will learn the sex of the unborn baby around May, but would be breaking an imperial household taboo to reveal anything at that stage.

Crown Prince Naruhito has produced no male heir and the ill-health of his wife, Princess Masako, makes the prospect of his doing so increasingly unlikely. Prince Akishino and Princess Kiko have two daughters, Kako, 11, and Mako, 14, and until yesterday it was assumed that their child-bearing days were behind them.

The fate of Mr Koizumi’s proposed amendments to the 1947 imperial law, which bars female descendents from the throne, is now in doubt.

Successive opinion polls show overwhelming popular support for the amendments, and pushing them through would have been a final legacy, Cabinet sources say. But Princess Kiko’s pregnancy leaves the plans in doubt.

Analysts said that if Mr Koizumi successfully pressed ahead with his proposed revisions and the child turns out to be a boy, the public would be left with the task of choosing between the legitimate heir under the old law and Aiko, a toddler for whom the country has enormous affection.

Tsuneyasu Takeda, a veteran royal commentator, said: “Now people know a baby is on the way, there is no time to create a new version of the Bill before the Diet session. The precondition of the argument for revising imperial law was that we were facing a succession crisis. But now we know we might get a baby boy so the crisis has not begun yet.”

Mr Koizumi defended his haste in pushing the Bill through the current parliamentary session. He said this was necessary so that Princess Aiko could be educated “with the awareness that she will one day assume the throne”.

But his contentious Bill has already unleashed a fierce revolt within the Cabinet, drawing open criticism from the Foreign and Finance Ministers.

More than 135 ruling party MPs have also expressed opposition to the move. They believe that it would corrupt an imperial line that is supposedly unbroken as far back as the Sun Goddess. Princess Kiko’s pregnancy provides eight months of breathing space.

The traditionalists do not have many suggestions in the event that there is no male heir, but they have time to produce some. Some have proposed, in a bizarre echo of history, that Prince Naruhito be introduced to concubines and encouraged to breed until a boy is born.

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