Lakes of water existed on Mars at a time when the planet was previously thought to be a frozen desert, new satellite images have shown.

A team of British-led scientists now believes 20 kilometre-wide lakes of melted ice were dotted around parts of the Martian equator three billion years ago.

No-one had expected to find evidence of a warm, wet climate capable of sustaining surface water on Mars during this period of the planet's history, known as the Hesperian Epoch.

Lakes, seas and rivers may have existed on the planet at an earlier time between 3.8 billion and four billion years ago, experts believe. But before the Hesperian Epoch, the planet was assumed to have lost most of its atmosphere and turned cold and dry.

The new high-definition images from the American space agency Nasa's Mars Reconnaissance Orbiter spacecraft show a number of lake-beds linked by channels that could only have been formed by running water. The dry lakes will be good places to look for signs of now-extinct microbial life, say scientists.

Dr Nicholas Warner, from Imperial College London, whose team analysed the images, said: "Most of the research on Mars has focused on its early history and the recent past. Scientists had largely overlooked the Hesperian Epoch as it was thought that Mars was then a frozen wasteland. Excitingly, our study now shows that this middle period in Mars' history was much more dynamic than we previously thought."

The British researchers, including scientists from University College London, examined several flat-floored depressions located above Ares Vallis, a giant gorge that runs for 2,000 kilometres across the Martian equator.