A ball has been reported missing from the mouth of a stone lion on display in Beijing, according to Beijing Youth Daily.

A pair of stone lions — exhibited at the Old Summer Palace, an imperial garden looted by Anglo-French forces in 1860 — originally each had two stone balls in their mouths.

A cultural relic preservationist surnamed Li found that one of the balls was missing from the mouth of one lion when he visited the exhibition recently, according to the report.

The cultural relics on display, mainly stone carvings, were placed under a simple wooden structure and surrounded by a low fence. Visitors could easily reach over the fence and touch the carvings.

The management committee of the Old Summer Palace is investigating the suspected theft.

The stone lions, originally ornaments in the garden, were moved to a courtyard house in Beijing's Xicheng district during the Kuomintang's rule and returned in 2010.

Countless cultural relics were looted during the late Qing Dynasty (1644-1911), but few were returned and most are now scattered worldwide.

The style of carving is a highly skilled craft. The balls and the lion were carved at the same time. The balls are movable in the lion's mouth, but cannot be taken out without causing damage.

Xu Lanting, deputy director of the criminal lawyers' commission of the All China Lawyers' Association, told China Youth Daily that deliberate damage of cultural relics could lead to a prison term of up to 10 years.

"If the ball was taken away deliberately, it would constitute a criminal offense, but if it was unintentional, the person would likely receive a warning," Xu said.

Li Lei contributed to this story.