Japan's Emperor Akihito has been diagnosed with prostate cancer, and will undergo surgery next month, Japan's Imperial Household has announced.

A biopsy revealing the presence of cancer cells had been performed after a series of "slightly worrisome" blood tests.

Doctors believe the cancer has not spread to other parts of his body, the Kyodo news agency reported, citing officials within the Imperial Household.

The 69-year-old emperor is expected to remain in hospital for a month after he is admitted in January.

New Year address

According to Japanese tradition, Emperor Akihito is Japan's 125th imperial sovereign in an unbroken line from Emperor Jimmu, who ascended the throne in about 660BC.

He took the throne in 1989 following the death of his father Hirohito, who had ruled for 62 years.

Hirohito died of duodenal papilla cancer.

Emperor Akihito is still expected to address the public in his traditional New Year appearance.