Her behaviour of late suggests she carried a foetus until late term, but lost it.
Chris West, chief executive officer for the Royal Zoological Society of Scotland said: "We are all saddened by this turn of events after so many weeks of waiting.
"Timings are difficult to pinpoint at this moment, but we had a meeting this morning where Tian Tian's behaviour and hormone results were reviewed and have come to the conclusion that it is very likely she has lost the pregnancy.
"Up until now, Tian Tian has consistently shown signs of pregnancy - she passed a mucus plug around mid-September and began producing colostrum.
"She also experienced a prolonged secondary-rise in progesterone.
"However, the veterinary team has noticed a significant decline in the amount of colostrum being produced and over the last few days she has returned to the normal eating and behavioural patterns of a non-pregnant panda."