a stable Imperial succession is an issue of extremely grave importance," he said.

Yukio Hatoyama, secretary general of the Democratic Party of Japan, the largest opposition party, said it is now a time for celebration, not debate.

"I only pray that Princess Kiko will steadily recover and the prince will grow up healthy," Hatoyama said.

Former Prime Minister Yasuhiro Nakasone said the issue of revising the law has "disappeared" for now, adding that the birth of a boy means there is no need to revise the Imperial House Law for the time being.