The Scotland zoo's chief executive, Chris West, said Tian Tian carried a baby until late in the pregnancy.
"We are all saddened by this turn of events after so many weeks of waiting," West said. "Timings are difficult to pinpoint at this moment, but we had a meeting this morning where Tian Tian's behavior and hormone results were reviewed and have come to the conclusion that it is very likely she has lost the pregnancy."
Tian Tian was artificially inseminated April 22 by sperm from two donors, The Guardian reported.
Her pregnancy was unusual, zoo officials said, and she was close to reaching the longest panda pregnancy at six months.
Panda pregnancies are very hard to track, but Tian Tian recently began eating and behaving normally, an indication she is no longer carrying the baby.
"Our dedicated team of keepers, veterinary staff and many others worked tirelessly to ensure Tian Tian received the best care possible," West said.