Located under a layer of Martian ice, the lake is about 12 miles (20 kilometers) wide, said the report led by Italian researchers in the US journal Science.

It is the largest body of liquid water ever found on the Red Planet.

"Water is there. We have no more doubt," co-author Enrico Flamini, the Italian space agency's Mars Express mission manager, told a press conference.

Mars is now cold, barren and dry but it used to be warm and wet. It was home to plenty of liquid water and lakes at least 3.6 billion years ago.

Scientists are eager to find signs of contemporary water, because such discoveries are key to unlocking the mystery of whether life ever formed on Mars in its ancient past, and whether it might persist today.