Scientists said Wednesday they had found life-giving chemicals in water at least 1.5 billion years old, which they are now combing for signs of microscopic organisms surviving from a prehistoric age.

The water, isolated in pockets deep underground for billions of years, is now pouring out of boreholes from a mine 2.4 kilometres (1.5 miles) beneath Ontario, Canada, they wrote in the journal Nature.

"This water could be some of the oldest on the planet and may even contain life," the team said in a statement.

Not only that -- the similarity between the rocks that trapped the fluid and those found on Mars raised hopes that similar life-sustaining water could be buried deep inside the Red Planet, they said.

"The findings... may force us to rethink which parts of our planet are fit for life," they added.

The British and Canadian researchers analysed the water and found it was rich in dissolved gases like hydrogen and methane that are able to sustain microscopic life not exposed to the sun for billions of years, as is the case on the ocean floor.

The rocks around the water were dated about 2.7 billion years old, "but no one thought the water could be the same age, until now," the team said.

Analysing the water's composition in the lab, the team estimated that it was at least 1.5 billion years old, possibly more.

"Our finding is of huge interest to researchers who want to understand how microbes evolve in isolation, and is central to the whole question of the origin of life, the sustainability of life, and life in extreme environments and on other planets," said Manchester University researcher and study co-author Chris Ballentine.

Before this discovery, the only other water from this age had been found trapped in tiny bubbles in rock, incapable of supporting life.

The Canadian water has characteristics similar to much younger water flowing from a mine 2.8 kilometres below ground in South Africa, which is known to support microbes.

"Our Canadian colleagues are trying to find out if the (Ontario) water contains life," said lead author Greg Holland of Lancaster University.

"What we can be sure of is that we have identified a way in which planets can create and preserve an environment friendly to microbial life for billions of years.

"This is regardless of how inhospitable the surface might be, opening up the possibility of similar environments in the subsurface of Mars."