A 12-mile long lake of liquid water has been detected under the polar ice caps of Mars, raising hopes that life could exist beneath the surface.

Scientists operating ground-penetrating radar on board the European Space Agency’s Mars Express spacecraft, which is orbiting the red planet, announced the finding on Wednesday, ending decades of speculation.

The huge body of water exists about one mile beneath the southern Martian ice cap and appears similar to Lake Vostok, which was found 2.4 miles beneath Antarctica and contained more than 3,500 species.

"This thrilling discovery is a highlight for planetary science and will contribute to our understanding of the evolution of Mars, the history of water on our neighbour planet and its habitability,” said Dmitri Titov, ESA's Mars Express project scientist.

"The long duration of Mars Express, and the exhausting effort made by the radar team to overcome many analytical challenges, enabled this much-awaited result, demonstrating that the mission and its payload still have a great science potential.”