Two sections of the Larsen ice shelf collapsed in 1995 and 2002. Now satellite measurements have confirmed that it has thinned by as much as 18 metres more than usual in the past decade, because of a warmer ocean.
The report comes a day after a University College London report in the journal Nature confirmed a 40% thinning of the ice in the Arctic Ocean in the past 30 years.
Andrew Shepherd of Cambridge University and colleagues from University College London, Bristol, and the Antarctic Institute of Argentina, report in the journal Science that they have been mapping changes in the Larsen shelf, on the east of the Antarctica peninsula, since 1992.
The shelf covers approximately 27,000 sq miles and is on average 300 metres deep. Its faster rate of melting is releasing an extra 21bn tonnes of icy water into the oceans each year. This is equivalent to eight times the annual flow of the river Thames.
Warmer oceans, as well as warmer air, are now thought to play an important part in the process.
The scientists used radar from a European satellite, accurate to within 20cm, to track the slow changes in the thickness of the shelf.
"We have shown that the Larsen ice shelf has progressively thinned due to the combined effects of surface and basal ice melting," Dr Shepherd said.
"The previously undetected imbalance may provide a simple link between the regional climate warming and the successive disintegration of of Larsen ice shelf sections, and suggests that fluctuations in the surrounding ocean should be considered in any future assessment of climate change."
The area of summer sea ice in the Arctic has been shrinking for decades. The thickness of the ice - originally about three metres - has been steadily declining.
The melting of the Arctic has been firmly linked with the pattern of global warming - rising air temperatures, thawing glaciers and longer northern summers - but the evidence in the Southern Ocean has been much more ambiguous.
"The ocean temperature rise parallels global temperature rises, whereas the atmospheric warming of the peninsula was 10 times greater," Dr Shepherd said. "In the Arctic, it has been fairly unequivocally related to atmospheric temperature changes. The Larsen shelf is much, much thicker, and had previously been thought to be disintegrating because of the atmosphere.
"It has always been difficult to see why the atmosphere should have such a great effect. But now it seems more likely that it is the ocean that is causing the change."