"There is an investigation taking place. There is no information as to how it happened," a spokesman told the Telegraph.

Sources have revealed that the cage door to the gorilla's enclosure was left open.

“Every enclosure has a gated area which is off-limits to the public to stop animals running straight past when the door is opened,” the source said.

“That door shouldn’t have been left open.”

The zoo, which describes the gorilla as a "gentle giant", has refused to confirm if this is the case.

The gorilla has now been returned to his enclosure.