Nasa scientists have produced the most compelling evidence yet that bacterial life exists on Mars.

It showed that microscopic worm-like structures found in a Martian meteorite that hit the Earth 13,000 years ago are almost certainly fossilised bacteria. The so-called bio-morphs are embedded beneath the surface layers of the rock, suggesting that they were already present when the meteorite arrived, rather than being the result of subsequent contamination by Earthly bacteria.

“This is very strong evidence of life on Mars,” said David Mackay, a senior scientist at the Nasa Johnson Space Centre , who was part of the team of scientists that originally investigated the meteorite when it was discovered in 1984.

In a 1996 study of the sample, Dr Mackay and others argued that the microfossils were evidence of life, but sceptics dismissed the claims, saying that similar-shaped structures might not be biological. The new analyses, the product of high resolution electron microscopy, make a strong case for the Allan Hills 84001 Meteorite having carried Martian life to Earth. The microscopes were focused on tiny magnetite crystals present in the surface layers of the meteorite, which have the form of simple bacteria. Some argued that these could be the result of a carbonate breaking down in the heat of the impact.

The new analyses show that this is very unlikely to have resulted in the kinds of structures seen in the rock. Close examination suggested that about 25 per cent of the crystal structures were chemically consistent with being formed from bacteria.

“We feel vindicated. We’ve shown the alternate explanation is absolutely incorrect, leading us back to our original position that these structures are formed by bacteria on Mars,” Dr Mackay said.

Dennis Bazylinski, an astrobiologist from the University of Nevada who peer-reviewed the findings, said: “Until now I was on the fence but this paper has really thrown out the non-biological explanation.” However, he added that the study was not a “smoking gun” for life on Mars. “One meteorite is never going to answer such a complex question,” he said.

According to scientists, the meteorite was broken off the surface of Mars by the impact of an asteroid, and reached Earth after floating through space for about 16 million years. It landed in Allan Hills in Antarctica. Scientists were able to trace the meteorite back to Mars, as its chemical composition matched the relative proportions of various gases measured in observations of the atmosphere of Mars made by the Viking spacecraft in the 1970s.

The meteorite also preserves evidence of liquid water on Mars, suggesting that the planet may have had more suitable conditions for life to develop in the past. The investigation was published in the November issue of Geochimica et Cosmochimica Acta, the journal of the Geochemical and Meteoritic Society. Nasa is expected to announce the findings formally on Monday.

The team has also been studying two other Martian meteorites — Nakhla, which landed in Egypt in 1911, and Yamato 593, which was found by a Japanese expedition to Antarctica. In research due to be published shortly, the scientists claim that both of these fossils also show evidence of microbial life.

Bill Clinton, then the US President, said of the research in 1997: “It speaks of the possibility of life. If this discovery is confirmed, it will surely be one of the most stunning insights into our Universe that science has ever uncovered. Its implications are as far-reaching and awe-inspiring as can be imagined.”