The giant cyclonic storm system is more than 1,600 km (1,000 miles) across. The eye of the storm is nearly 200 miles in diameter.

Astronomers say this makes the storm nearly four times the size of the US state of Texas.

It is composed of water-ice clouds like storm systems on Earth, rather than the dust typically found in Martian storms.

The system is similar to so-called "spiral" storms observed more than 20 years ago by Nasa's Viking Orbiter spacecraft. But it is nearly three times as big as the largest Martian spiral storm system seen before.

MGS saw whispy clouds when it passed over the same area

The Viking discovery was made during the same stage in the Martian year as the Hubble find.

"These rapidly growing and decaying systems do appear to be typical of the Martian polar weather at this season, which is northern mid-summer," says Jim Bell, assistant professor of astronomy at Cornell University.

"The storm we detected from Hubble appears anomalous because of its size, but I wouldn't be surprised if the Mars Global Surveyor (MGS) spacecraft sees other storms of this size eventually as well."

The MGS flew over the region three days after Hubble took its pictures and captured just normal cloud patterns for this time of year.