Scientists claimed last night to have found proof that there was once life on Mars.

They presented new evidence which they say removes any doubt that a meteorite carried the remnants of ancient Martian bacteria to Earth.

The findings, from the US space agency Nasa, indicate that Mars may once have been teeming with bugs which lived at the bottom of shallow pools and lakes.

They also suggest there would have been plants or organisms capable of photosynthesis and complex ecosystems on Mars.

However, British experts said the evidence, though exciting, had to be treated with caution.

The work was carried out by a team led by Nasa scientist Dr Imre Friedmann that has been studying the 4.5 billion-year-old meteorite ALH84001.

Scientists think the potato-sized rock, found in Antarctica's Allan Hills ice field in 1984, was blasted off the surface of Mars by a comet or asteroid 15 million years ago.

After drifting through space, it was caught by the Earth's gravity and landed in Antarctica, where it lay for 13,000 years.

In 1996, Nasa rocked the scientific world by announcing the discovery of worm-like ''microfossils'' left by Martian bacteria within the meteorite.

Oher experts have argued that non-biological chemical processes could explain what was found. But Dr Friedmann insists his team has now found clinching evidence that should silence the doubters.

Inside a small fragment of the meteorite, he says, they detected chains of magnetic iron crystals which could only have been left by living organisms.

Similar magnetite crystal chains are seen in Earthly bugs called magnetotactic bacteria