Edinburgh Zoo's female giant panda Tian Tian lost her baby cub in the late stages of pregnancy, animal experts believe.
The zoo has announced that Tian Tian is no longer pregnant despite showing signs she was to mother a cub up to last week.
The Royal Zoological Society of Scotland (RZSS), which runs the attraction, believes she was successfully inseminated in April but lost the foetus at late term.
Chief executive Chris West said: "We are all saddened by this turn of events after so many weeks of waiting.
"Such a loss has always been in our minds as a very real possibility, as it occurs in giant pandas as well as many other animals, including humans.
"Our dedicated team of keepers, veterinary staff and many others worked tirelessly to ensure Tian Tian received the best care possible, which included remote observation and closing the panda enclosure to visitors to give her quiet and privacy."
Tian Tian (Sweetie) had been keeping the zoo, and the public, guessing over her possible pregnancy since she was artificially inseminated on April 21.
In August experts noted signs that she had been successfully fertilised and it was hoped a panda cub would be born by September.
Last week they said Tian Tian had started to produce colostrum and preparations were being made for the much-anticipated arrival.
A more recent review of her behaviour and hormone levels have led experts to believe she lost the foetus.
"Over the last few days she has returned to the normal eating and behavioural patterns of a non-pregnant panda," said Mr West.
Tian Tian and male panda Yang Guang (Sunshine) arrived at Edinburgh Zoo from China in December 2011.
Zoo bosses hoped the pair would mate naturally when she came into season.
Experts ruled out putting them together after assessing her behaviour and Tian Tian was artificially inseminated using semen from Yang Guang and frozen semen from Bao Bao, a ''genetically important'' panda which died in Berlin Zoo last year.
The zoo said "everything possible" had been done to give Tian Tian the best care during her pregnancy.
The animals' enclosure will remain closed until the end of the week.
Mr West said: "We are conducting a detailed review of the scientific data collected, but I am totally confident that we did everything it was possible to do.
"The majority of research centres and zoos with giant pandas around the world have not successfully bred until the third or fourth year and what we have achieved, considering we have had giant pandas for less than two years, is immense.
"New hormone research is beginning to indicate that lost pregnancies are more common in giant pandas than first thought, though at the moment no one knows why."
Female pandas only ovulate once a year, giving a window of 36 hours in which they can get pregnant.
Edinburgh-based charity OneKind is opposed to breeding wild animals in captivity and criticised the zoo for the lengths it has gone to to intervene "in what should be a natural process".
Chief executive John Brady said: "Edinburgh Zoo has gone through a rigorous and extensive process in a bid to bring a panda cub to Scotland and one needs to ask whether or not this was in the panda's best interests.
"There are questions around the morals of the zoo's breeding programme which must be asked in light of the developments with Tian Tian and her pregnancy which she endured under an intense media and public spotlight.
"While we are extremely saddened by the possibility of a miscarriage and concerned for the wellbeing of the panda at this time, we need to think about the life a panda cub would have had at the zoo, being unable to forage, feed and roam free as nature intended."
Speaking at the zoo, director of the panda project Iain Valentine said: "What we can say is that when we look at all the data she did conceive, she did get pregnant, and what we're trying to do now is pinpoint where she may have lost her pregnancy.
"That is quite tricky to do with the information we have got.
"Why does this happen? We don't truly understand, but what we do know is this is a species which lives on the edge of a balance between nutritional intake and reproducing.
"If things aren't quite right this can be a reason why they absorb the young and cancel the pregnancy.
"We are determined to learn from this experience and if we can improve then we will do."