The first tiger cub to be born at London Zoo for 17 years has died.
Keepers are said to be distraught after the two-week-old cub was discovered on the edge of the pool inside the tigers' enclosure.
"They work very closely with the tigers, so it is incredibly sad news for them," said a zoo spokeswoman.
When five-year-old Sumatran tiger Melati gave birth to the cub the event was recorded on cameras hidden in the den.
Keepers raised the alarm in the early hours of Saturday when they could not see the cub on CCTV.
The spokeswoman said a post-mortem on Sunday confirmed the cub had drowned.
Melati is thought to have carried the cub outside, but keepers are unclear as to how the cub got into the pool as there are no cameras in the wider enclosure.
Curator Malcolm Fitzpatrick said: "We're heartbroken by what's happened. 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.
"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.
"They are devoted to those tigers and are distraught."
Melati's pregnancy lasted around 105 days and was kept secret by the zoo. The keepers kept a careful watch on the first-time mother through the cameras so they would not disturb her.
Meanwhile, Edinburgh Zoo has said the UK's female giant panda, Tian Tian, has lost her foetus. It said she had been successfully inseminated, but lost the foetus at late term.
A statement said: "All of her hormonal and behavioural signs now indicate that she had conceived and carried a foetus until late term, but then lost it."