Space bugs? Other scientists think they probably come from Earth
"They look like terrestrial cells but it's exceedingly unlikely that they could have been lifted up from the ground in the quantity that we find - something like 100 bacteria per litre of air," Professor Wickramasinghe said.
But scientists working in the field of astrobiology - the study of life in the Universe - said they had yet to be convinced by the Cardiff evidence.
Clusters of cells
Professor Wickramasinghe outlined the findings of joint UK/Indian research at a meeting of the International Society for Optical Engineering in San Diego, California, US.
Balloons were sent up from a research facility in Hyderabad to collect samples of air at high altitudes. The samples were then sent to Cardiff where microbiologists were able to isolate clusters of cells.
"My colleagues have identified these as living cells because they take up a certain dye and show up as a fluorescent spot on the microscope," Professor Wickramasinghe said.
"But the team haven't yet been able to culture them and that gives added confidence that the microbes are something alien. Because, if they were ordinary run-of-the-mill bacterial contaminants, they would be very easy to culture."
The professor has long argued that comets and space dust probably brought the seeds of life to Earth. He and the eminent astronomer Sir Fred Hoyle put forward the so-called Panspermia Theory, which suggests that life, or the building blocks of life, could be carried to planets by comets or drifting interstellar dust particles.
Need for publication
However, UK scientists interested in the search for extraterrestrial life said they would need to see more hard evidence from the experiments before they could accept the analysis.
Dr Alan Penny, an astronomer from the Rutherford Appleton Laboratory, and a member of the UK Astrobiology Forum and Network, told the BBC that "there were unresolved questions about contamination in the experiments". "I think Chandra is being a bit optimistic," he added.
Experimental error was also highlighted as a potential problem by Professor John Zarnecki, from the Open University, another network member.
"I would like to be positive but the burning question for me is: how do we know there is no contamination? Let's see the work published. Let's see it peer reviewed," he told BBC News Online.
And Professor George Tranter, a biochemist from Imperial College, said: "We need to see the evidence. They have long-standing theories, which they're looking to find evidence to support."