The announcement comes from a team at the US space agency, Nasa, who have been studying meteorites that have fallen to Earth from our nearest planetary neighbour.
It is understood that the scientists discovered the fossil of a long- extinct life form - almost certainly a very simple, single-celled organism. It probably would have lived in water trapped below the surface of the planet, millions of years ago. If their findings, to be published next week in the Science journal, are confirmed, it would provide new evidence that Mars was once able to sustain life.
The news will also give extra impetus to Nasa's next Mars mission, due to be launched this November. In 1976, the laboratory on its Viking spacecraft found no trace of living organisms on the Martian surface.
The new discovery was embedded in a melon-sized meteorite found in 1984 in Antarctica. It is one of 12 sizeable meteorites whose composition indicates that they originated in volcanic rock on Mars. The organism would have lived in the water that is thought to have once been plentiful on the planet's surface - as indicated by the number of dry "river beds". The organism could have been trapped in the rock during a volcanic eruption that might have flung it from the planet's surface, or been knocked into space by a meteor strike. The huge pressures exerted on it would kill and fossilise it, preserving its outline and micro-structure in the rock around it.
"If they have found something, then it's absolutely fascinating and has huge ramifications for us," said Monica Grady, researcher in meteorites at the Natural History Museum.
"The oldest rocks on Earth also show microfossils, where bacteria have been captured."
Last night Science refused to comment on the paper ahead of publication. However, a press conference by the paper's authors is understood to have been arranged for next week.
Earlier this year, an international gathering in London of planetary scientists aired views that life could have originated on the surface of Mars at the same time as on Earth, roughly 3.8 billion years ago.
But as the planet began to move farther from the Sun, the water moved underground - as would any organism living in it. Scientists at the conference reckoned that any living organisms would retreat to the planet's interior, seeking the warmth of hot springs heated by Martian volcanoes, to get the energy to sustain life. Given that life forms have been found which can survive in such volcanic springs on Earth - in the form of "white worms" - this seemed a viable theory.
Jack Farmer, a senior scientist at Nasa's Ames Research Centre, said present conditions were "not conducive for life as we know it". But the conference reckoned that life could have existed between 100 metres and 1,000 metres below the surface.
It is possible though that the Nasa finding is the result of an error caused by contamination of the meteorite after it reached Earth.
It might be possible that ancient terrestrial life could have got into the meteorite, which would be extremely hot after its passage through the atmosphere, and then been trapped there when the rock cooled and hardened.
Dr Grady said such contamination was possible. "This would have been on the surface of Antarctica for thousands of years, carried by the ice and exposed to the wind. Those might have contaminated it. But thousands of meteorites from been found there, and there's never been a similar find."Reuse content