Palace officials said 14-year-old Princess Aiko has been staying home since September 26, after complaining of stomach problems and dizziness attributed to fatigue from studying for exams and practising for a school athletic event.

Aiko is the only child of Crown Prince Naruhito, next in line to the Chrysanthemum Throne, and his wife Masako, who has been suffering from a stress-related mental condition for more than 10 years.

Officials say Nobutake Odano, the grand master at Naruhito's palace, told reporters on Tuesday that Aiko is recovering but needs more time before returning to school. A palace hospital health check earlier this month found no major problem.

Aiko, known as the princess who rarely smiles, had occasionally missed class while in elementary school. Over the summer, she was seen waving at crowds as she accompanied her parents on hikes and at other public events.

Masako cancelled a planned out-of-town official appointment this month due to fatigue and concern over having to take care of her daughter, the palace said.

A Harvard graduate and former diplomat, Masako has had stress-related conditions since soon after giving birth to Aiko, following criticism for not producing a boy.

Concerns about a shortage of male successors have been rekindled after Akihito, 82, conveyed his apparent abdication wish in August, citing his old age and health. Japan's Imperial House Law does not allow female emperors.