The Kangerdlugssuaq glacier on Greenland's east coast is one of several that drains the huge Greenland ice sheet. The glacier's movements are considered critical in understanding the rate at which the ice sheet is melting.

Kangerdlugssuaq is about 1,000 metres (3,280ft) thick, about 4.5 miles wide, extends for more than 20 miles into the ice sheet and drains about 4 per cent of the ice from the Greenland ice sheet.

Experts believe any change in the rate at which the glacier transports ice from the ice sheet into the ocean has important implications for increases in sea levels around the world.

If the entire Greenland ice sheet were to melt into the ocean it would raise sea levels by up to seven metres (23ft), inundating vast areas of low-lying land, including London and much of eastern England.

Computer models suggest that this would take at least 1,000 years but even a sea-level rise of a metre would have a catastrophic impact on coastal plains where more than two-thirds of the world's population live.

Measurements taken in 1988 and in 1996 show the glacier was moving at a rate of between 3.1 and 3.7 miles per year. The latest measurements taken this summer show it is now moving at 8.7 miles a year.

Gordon Hamilton, professor of earth sciences at Maine University, who made the measurements using global positioning system (GPS) satellites, said the velocity measurements were accurate to within about 45 metres of movement per year and that Kangerdlugssuaq is probably the fastest-moving glacier in the world.

"This is a dramatic discovery. There is concern that the acceleration of this and similar glaciers and the associated discharge of ice is not described in current ice-sheet models of the effects of climate change," Professor Hamilton said.

"These new results suggest the loss of ice from the Greenland ice sheet, unless balanced by an equivalent increase in snowfall, could be larger and faster than previously estimated."

Kangerdlugssuaq is a relatively southern glacier and the scientists are concerned that more northerly glaciers, which have not shown such an increase in speed of movement, may follow suit if the warming of the Arctic region continues.

Professor Hamilton said: "As the warming trend migrates north, glaciers at higher latitudes in Greenland might also respond in the same way as Kangerdlugssuaq glacier.

"In turn, that could have serious implications for the rate of sea-level rise," .

The independently funded scientists from Maine made the measurements from the Greenpeace ship Arctic Sunrise which is sailing off the Greenland coast documenting the signs and impacts of climate change in the Arctic.

Martina Krüger, Greenpeace's expedition leader, said there were signs that the glacier was both retreating and moving faster at the same time.

"The speed up of the glacier is due to increased surface melting, which generates meltwater that trickles down to the glacier bed and lubricates the bedrock," Ms Krüger said.

"This decreases the friction and the glacier slides more easily. This is accompanied by a thinning of the glacier, because the glacier stretches more.

"The front or terminus of the glacier is usually pinned or anchored to a higher feature on the ocean floor, such as a ridge from where it calves icebergs.

"As the glacier thins, it will float off that pinning point. Once the glacier has lost that anchoring, it retreats into deeper water behind that pinning point."