A keeper feeds Pan Pan at the Dujiangyan base of the China Conservation and Research Center for Giant Panda on Dec 21, 2016. [Photo/Xinhua]

Pan Pan passed at 4:50 am. An autopsy is being carried out to find the cause of his death, though the circumstances are not thought to be suspicious.

"Pan Pan was the equivalent to about 100 human years, but he had been living with cancer and his health had deteriorated in the past three days," Tan Chengbin, a keeper with the Dujiangyan base of the China Conservation and Research Center for Giant Panda, told Xinhua. "He had lost consciousness."

Though born in the wild in Sichuan's Baoxing county in 1985, Pan Pan lived in captivity from just a few months old.

"Panda grandpa," as he became known, was diagnosed with cancer in June this year, and also suffered from common old-age conditions, such as cataracts and poor teeth.

A Xinhua reporter who last saw him on Dec 21 said he was in a "nursing home" for elderly pandas, and though he was very thin he was keeping in good spirits.

"When the keeper called his name and offered him food, usually steamed corn bread or fresh bamboo leaves, he was able to respond and move out to have dinner," the reporter said.

Pandas are notoriously difficult to breed, but Pan Pan lived a particularly active life for a panda and fathered many cubs over the past 20 years. He has more than 130 descendants, accounting for 25 percent of the world's captively bred panda population.

The average lifespan of wild pandas is normally 20 years, but those in captivity usually live longer.

Pan Pan, which means "expectation" in Chinese, was also name of the mascot for the 1990 Beijing Asian Games, though the mascot was not modeled on "panda grandpa" but on a 36-year-old female panda Basi, currently the oldest panda in the world.

Giant pandas are one of the world's most endangered species. Fewer than 2,000 pandas live in the wild, mostly in the provinces of Sichuan and Shaanxi. At the end of 2013 China had 1,864 giant pandas in the wild. There are also 422 in captivity, China's State Forestry Administration said.

Pan Pan is survived by numerous descendants.