Edinburgh Zoo has rejected claims that its panda enclosure is unsuitable for breeding and that Tian Tian was never pregnant.
Dr Gareth Starbuck, an animal breeding expert at Nottingham Trent University, criticised the £275,000 home.
He said it was an error to keep Tian Tian and Yang Guang in sight of each other, as it failed to replicate conditions in the wild.
Criticism has also come from San Diego Zoo, which said it was “unlikely” that Tian Tian was ever pregnant.
San Diego’s 20-year-old giant panda Bai Yin gave birth to her sixth cub in July 2012.
A spokeswoman from the American attraction: “Pandas do not struggle with pregnancy. They reproduce without difficulties if given what they need to do so.
“Artificial insemination has a very low success rate. So it is unlikely that Tian Tian was pregnant. We are unaware of any information indicating that a pregnancy had been confirmed through ultrasound visualisation of the foetus.”
But Edinburgh Zoo said the claims by San Diego experts were groundless. A spokeswoman said: “San Diego Zoo has never seen the scientific data generated surrounding Tian Tian’s pregnancy.
“We have worked closely with our Chinese colleagues and colleagues from other American giant pandas zoos to analyse the data and they all concur with our findings that Tian Tian was pregnant.”
Tian Tian was artificially inseminated on April 13 after she and her intended partner, Yang Guang, failed to mate.
The zoo announced in August that she was pregnant and it was thought she would give birth by the end of the month. However, it was confirmed earlier this week that there would be no cub. Dr Starbuck, 42, who specialises in giant panda behaviour and reproduction, said it was “possible” Tian Tian was never pregnant.
He said: “The best way to get an animal to behave in a successful manner is to recreate their natural environment as best as possible.
“The panda enclosure in Edinburgh is sparse, has a lot of concrete, one token tree and not much cover. In the wild they have plenty of cover.
“We put them next to each other and expect them to mate when in the wild they are miles apart and only come into contact when she is ready to mate. Why is she going to be interested in the boy next door?”
The zoo spokeswoman said: “The panda enclosures were designed in collaboration with Chinese experts who are the foremost authority. Both pandas have large separate outdoor enclosures that contain grass, many trees, large climbing frames, ponds and caves.
“Although pandas are solitary animals, Chinese experts say it is important that male and female pandas understand there is another panda of the opposite sex close by.”