A spokeswoman for the zoo, which has over 7,000 animals, said: “In November Manilla showed some subtle changes to her behaviour and keepers alerted our in-house vet team.

“She showed no physical signs of being pregnant and, as she had not been with a male between July and October, she was well outside the known birth window for this species.

“The birth of her litter and the subsequent incident with the male was, therefore, completely unforeseeable.”

She added the incidents had been “distressing” for staff but defended the zoo by saying it had the “highest standards of animal welfare, education and conservation in Europe, as well as world-class in-house veterinary care”.

She added: “We actively encourage natural animal behaviours and group dynamics.

“Our animals are also housed in natural enclosures, replicating environments they would be used to in the wild.

“As a result, on rare occasions, despite our best efforts, we are not able to prevent unfortunate and unforeseen situations from occurring.”

The British and Irish Association of Zoos and Aquariums also stood up for the establishment.

A spokesman said: “Staff caring for these animals have been understandably distressed after these sad and unusual events.

“But we are confident that Bristol Zoo is doing everything possible to ensure that similar situations will not arise again in the future.”