Japan's former emperor Akihito has been diagnosed with heart failure but is in stable condition after being prescribed medication, the Imperial Household Agency said Tuesday.

The diagnosis was made during an unrelated health checkup last month on the 88-year-old, who in 2019 became the first member of the world's oldest royal family to abdicate the throne in more than 200 years.

The examination confirmed heart expansion and pleural fluid, an agency spokesman told AFP.

A subsequent checkup involving a MRI scan at a Tokyo hospital found the retired emperor has right heart failure.

"He is receiving treatment now and he is convalescing," the spokesman said.

In 2020, the former emperor temporarily lost consciousness and collapsed at his residence, but medical checkups found "no abnormalities".

He has also suffered temporary cerebral anaemia, a condition involving insufficient blood supply to the brain, as well as prostate cancer and other heart problems.

The popular former monarch shocked the country in 2016 when he signalled his desire to take a back seat after nearly three decades on the Chrysanthemum Throne, citing his age and health problems.

In 2019, his son Naruhito became the new emperor, ushering in the new imperial era of Reiwa or "beautiful harmony."

tmo/sah/dhc