Prince Katsura, a cousin of Emperor Akihito and sixth in line to the Chrysanthemum Throne, died at a Tokyo hospital at 10:55 a.m. Sunday, the Imperial Household Agency announced. He was 66.

The prince had been rushed to University of Tokyo Hospital in a critical condition earlier Sunday, but died of acute heart failure, the agency said.

As the second son of Prince Mikasa, a 98-year-old uncle of the Emperor, Prince Katsura had been sixth in line to the throne. The death of the prince, who was not married and had no children, brings an end to his household.

Prime Minister Shinzo Abe released a statement saying, “I cannot help but be saddened by His Royal Highness Prince Katsura’s death.”

Abe said the prince had made contributions in various fields, including agriculture and traditional crafts, and played a major role in building international goodwill, for instance by heading the Japan Australia New Zealand Society, an association dedicated to deepening the friendship among the three countries.

The prince had been in and out of hospital in recent years due to a variety of illnesses, including blood poisoning and possible infectious diseases since he suffered an acute subdural hematoma, a condition in which blood gathers around or inside the brain, in 1988.

His death is the first of an Imperial family member since June 2012, when Prince Tomohito of Mikasa, the prince’s elder brother, died at age 66 after a long battle with throat and tongue cancer.

Born on Feb. 11, 1948, Prince Katsura graduated from Gakushuin University in 1971 and pursued his studies at a graduate school of the Australian National University.

After returning to Japan, he worked part time at NHK from 1974 to 1985. He established his own household within the Imperial family in January 1988, becoming its first unmarried member to do so since the end of World War II.