"If it is true," said Sadame Kamakura, the Agency's Grand Steward, "there is no happier news." Keizo Obuchi, the Prime Minister, called it "a cause for celebration", and a succession of guests on the daytime chatshows expressed their rapture. Confirmation will come only with further tests next week, but there seems to be little doubt.
The news has been greeted with more than the usual gurgles provoked by royal births for the simple reason that it has been so long in coming. The royal couple married six years ago; he is 39 and she is 36. Japan has no equivalent of the British tabloids but, in their discreet way, the country's women's magazines have kept up a drum roll of anticipation for yesterday's announcement.
Hardly a month has passed without articles pointing out the couple's failure to produce an heir. Whenever the princess cancelled an engagement or appeared to be under the weather, hopes soared - only to be dashed when no announcement was forthcoming. At his annual birthday press conference, the prince has given increasingly solemn answers when questioned about his prospects for an heir. "I am afraid too much disturbance could upset the stork's mood," he said in 1994. This year, there were no jokes. "I fully recognise people's interest in this, and the importance of the matter," he said.
After yesterday's news was scooped by the Asahi newspaper, magazine editors were frenziedly working on special editions. "We are so glad to hear this news," said Yoshiya Yokota, the editor of Woman's Own, which is planning more than 20 pages of imperial pregnancy analysis in its next issue. "This will have a powerful effect of Japanese society."
The prince and princess are not the only Japanese couple to leave it a little late in producing a family. Japan has one of the lowest birth rates in the industrialised world, with the average mother bearing only 1.39 children. "There are many couples who don't have babies these days," says Ms Yokota. "When they see how happy the Imperial Family is, the number of children will increase."
The focus now will shift to the gender of the baby for, despite an unbroken line which is traced back to the Shinto sun goddess, the Imperial Family has a worrying shortage of boy children. Prince Naruhito's younger brother, Fumihito, has two children, but both are girls, ineligible to succeed to the throne. In fact, no member of the Imperial Family has given birth to a son for 30 years.