Two ice shelves on the Antarctic peninsula are crumbling far faster than anyone had predicted.

The shelves, Larsen B on the eastern side of the peninsula and Wilkins on the southwest, have together lost nearly 3,000 sq km in the last year.

The British and US scientists who made the discovery say the two shelves are in "full retreat".

They are based at the British Antarctic Survey in Cambridge, UK and the National Snow and Ice Data Center at the University of Colorado at Boulder in the US.

Despite plans to publish their findings in a scientific journal, the researchers have decided to release them now because of their alarm at what they found.

Satellite photographs show that Larsen B has continued to crumble after an initial small retreat in spring 1998.

Taken by surprise

Since last November the shelf, which is at least four centuries old, has lost a further 1,714 sq km.

The Wilkins shelf has lost almost 1,100 sq km in the last year.

The scientists had expected the break-up to happen, but more gradually.

Summer average temperatures will soon be above freezing

Ted Scambos, of NSIDC, said: "It happened much faster than we thought."

"It was nearly as much activity in a single year as we've seen in 10 or 15 years up to now on average."

David Vaughan, of BAS, said that over the last 50 years the shelves had lost about 7,000 sq km.

"To have retreats totalling 3,000 sq km in a single year is clearly an escalation."

The scientists believe the retreats are caused by a regional warming trend, which has caused the annual melt season to lengthen by two to three weeks over the past 20 years.

BAS says the average annual temperature in the region has increased by about 2.5 degrees C since the 1940s.

Forced apart

The average summer temperature is now approaching just over zero degrees Centigrade - the melting point of water.

Ice shelves are up to 300 metres thick, and warming at the surface will not itself cause them to melt.

Ice shelves help to protect glaciers from melting

But the scientists think the water which does melt enters cracks in the shelf and makes them wider.

Ted Scambos described the result as "an ice shelf that is essentially shattered, already being swept out. There are thousands of relatively small icebergs."

The shelves float on the ocean surface, so they do not cause sea levels to rise when they crumble.

But Ted Scambos said the glaciers behind the shelves could melt more quickly if they lost the shelves' protection.

"Other ice shelves have huge glaciers behind them, and large areas of ice to drain that are continental."

And melt water from the glaciers would add to sea level rise.

The US Geological Survey says global sea levels have risen by about 10cm (four inches) in the last century.