Eight penguins have been attacked and killed by an "urban fox" that broke into their water enclosure at Chessington World of Adventures.
Assistant zoo manager Lisa Britton said it "infiltrated" their home at Penguin Bay during the night.
Five adult and three infant Humboldt penguins died in the attack. A ninth was injured but made a full recovery after treatment from a vet.
The remaining 20 penguins have since been moved to a secure location.
Ms Britton said staff at the zoo, which has only just released details of the June attack, were "shocked and saddened" by the loss.
"We are investigating why this happened, as Penguin Bay, only built in 2015, had special measures put in place specifically to deter foxes," she said.
Penguin Bay has been closed while additional security measures are undertaken.
A note on the adventure park's website reads: "Our Humboldt penguins are currently enjoying their other home behind-the-scenes while we make alterations to Penguin Bay."
Chessington World of Adventures features a theme park with more than 40 rides as well as a zoo and sea life centre.