The UK's only female giant panda will not give birth to a cub this year, experts at Edinburgh Zoo believe.
Tian Tian was artificially inseminated for the third time earlier this year and vets said she conceived but did not know for definite if she was pregnant.
The zoo said the pregnancy window has now passed and it is believed Tian Tian "resorbed her pregnancy in late term", as is common among giant pandas.
Chris West, chief executive of the Royal Zoological Society of Scotland, said: "Based upon our scientific data, the window has now passed during which Tian Tian would have given birth; therefore RZSS has to advise that we now do not believe that our female giant panda will have a cub.
"Tian Tian is still showing behaviour of a pregnant panda, being sleepy and off her food, but we now must assume she has resorbed her pregnancy in late term.
"It is believed that resorption is a common occurrence in giant pandas, as it is in other species, and may well be the true explanation behind many so-called 'fake' pregnancies."
A team of three vets and a panda expert from China carried out the insemination on Tian Tian in March.
The procedure had been carried out in each of the last two years but she has so far failed to produce a cub.
Panda reproduction is a notoriously tricky process, with females only ovulating once a year.
The gestation period is typically five months and one or two cubs are usually born.
Mr West said the zoo had carried out "the world's most comprehensive hormone analysis of an individual female giant panda" as they tracked any potential pregnancy.
"We are also hopeful that the Royal Zoological Society of Scotland (RZSS) has made some key discoveries relating to giant panda pregnancy, which will add to the global understanding of this endangered species," Mr West said.
"The conservation of giant pandas is a complex international effort. Although still early in the birthing season, this year we have had fantastic news from the National Zoo, Washington, and Zoo Negara in Malaysia.
"Our hopes and best wishes are now with Memphis Zoo and Ocean Park Hong Kong. No giant panda zoo works in isolation and success for one institution means success for the overall giant panda conservation programme.
"Going forward we will conduct an exhaustive review and consult with other colleagues. RZSS is committed long term to giant panda conservation and working with our many research partners and colleagues in China."
There were hopes that a panda cub would be born last year when Tian Tian became pregnant following insemination, but vets who closely monitored her believe she may have resorbed the foetus on that occasion as well.
Tian Tian (Sweetie) and male Yang Guang (Sunshine) were the first giant pandas to live in the UK for 17 years when they arrived on loan from China in December 2011.
The panda enclosure at Edinburgh Zoo was closed to the public last week as keepers prepared for the arrival of a rare cub and is due to reopen today.