The scientists sent a balloon 27km into the stratosphere, which came back carrying small biological organisms which they believe can only have originated from space.
Professor Milton Wainwright said he was "95 per cent convinced" that the organisms did not originate from earth.
"By all known information that science has, we know that they must be coming in from space," he said. "There is no known mechanism by which these life forms can achieve that height. As far as we can tell from known physics, they must be incoming."
Some of the samples were captured covered with cosmic dust, adding further credence to the idea that they have originated from space.
"The organisms are not usual," said Professor Wainwright, who works at the University of Sheffield's Department of Molecular Biology and Biotechnology. "If they came from earth, we would expect to see stuff that we find on earth commonly, like pollen."
"We're very, very confident that these are biological entities originating from space," he said, acknowledging that absolutely certainty is hard to achieve in science.
The team believes that the entities are coming from comets, which are big balls of ice shooting through space. The samples were collected during a meteorite shower from a comet. As they hit the earth's atmosphere, the comets melt - ablate, to give it a technical term - releasing the organisms as they break down.
"The particles are very clean," added Prof Wainwright. "They don't have any dust attached to them, which again suggests they're not coming to earth. Similarly, cosmic dust isn't stuck to them, so we think they came from an aquatic environment, and the most obvious aquatic environment in space is a comet.
"They're very unusual beasts, not your normal kind of life from earth."
The organisms are probably not alive, but, excitingly, probably do contain DNA. Similar ones harvested during an earlier experiment have contained the chemical, which is one of the fundamental building blocks of life on earth.
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