The first Sumatran tiger cub to be born at London Zoo for 17 years has drowned in a pool of water, the zoo said on Tuesday.
Five-year-old Sumatran tiger Melati gave birth to the unnamed cub three weeks ago after a six-minute labour.
Zookeepers were "very distraught" after the cub from the critically endangered subspecies was discovered on the edge of the pool in the tiger enclosure on Saturday morning, a spokeswoman said.
"The keepers are naturally very distraught. They work very closely with the tigers, so it is incredibly sad news for them," she said.
A post-mortem found the tiny feline—which was too young to be named or sexed—had drowned.
The mother tiger was thought to have carried the cub outside the den but it was still unclear how the tiger cub drowned as there were no cameras in the outside enclosure.
London Zoo opened a new, bigger tiger enclosure earlier this year.
Curator Malcolm Fitzpatrick said the zoo staff were "heartbroken", adding: "To go from the excitement of the birth to this in three weeks is just devastating."
"Melati can be a very nervous animal and we didn't want to risk putting her on edge by changing her surroundings or routines, in case she abandoned or attacked the cub," Fitzpatrick said.
"At the time we thought it was in the best interests of Melati and her cub to allow her continued access to the full enclosure as normal.
"We would do anything to turn back the clock, and nobody could be more upset about what's happened than the keepers who work with the tigers every day."
The cub's birth on on September 22 was captured by hidden cameras, adding to the excitement about its arrival.
Melati's 105-day pregnancy was kept secret by nervous zoo staff.
The Sumatran tiger, a subspecies whose natural habitat is the jungles of Sumatra, Indonesia, is now classified as critically endangered on the International Union for Conservation of Nature's red list of threatened species.
The current wild population is estimated at just 300, down from around 1,000 in the 1970s, and the remaining animals are threatened by poachers, habitat loss and human conflict.
Explore further: Three rare Sumatran tiger cubs born at Indonesia zoo