Edinburgh Zoo has warned that its female giant panda, Tian Tian, may have lost her cub.
Hopes were high that Tian Tian, or Sweetie, would give birth at the end of August following artificial insemination in April.
But the zoo's panda expert said on Monday that Tian Tian was "now past her due date", and hormone tests showed that "something may be amiss".
"The evidence suggests that this may be bad news," said Iain Valentine at the Royal Zoological Society of Scotland.
He said there was still a chance the panda would give birth, and experts were monitoring her closely.
Many thought it could be third time lucky for the panda, which failed to mate with male companion Yang Guang, or Sunshine, despite encouragement from zoo keepers. She became pregnant last year, also after artificial insemination, but appeared to have suffered a miscarriage late term.
Giant pandas have difficulty breeding and their pregnancies are difficult to follow. Their fetuses are tiny and hard to detect, and the animals also experience "pseudo-pregnancies" during which behaviour and hormonal changes indicate they are pregnant even when they are not.
But scientists believed that Tian Tian was indeed pregnant and was likely to carry to full term, Valentine said.
Tian Tian and Yang Guang, both 11, are Britain's only pandas. They arrived from China in 2011.