Exeter (England), Feb. 2: Vast ice blocks are slowly collapsing into the sea off Antarctica, increasing the threat from the rising level of the world?s oceans, British scientists said yesterday.

Their discovery that the west Antarctic ice sheet is unstable overturns the previous international consensus that it would take 1,000 years for the floating ice to respond to rising temperatures.

Scientists from the British Antarctic Survey said that far from gaining in mass, as expected, because of increased snowfall in the polar regions, the west Antarctic ice sheet was losing 250 cubic km of ice a year. This means Antarctica is contributing at least 15 per cent of the current 2 mm annual rise in sea levels. Prof. Chris Rapley, director of the survey, told an international conference on climate change in Exeter called by Tony Blair: ?The previous view was that the west Antarctic ice sheet would not collapse before 2100. We now have to revise that judgment. We cannot be so sanguine.?

Four years ago, he said, the UN?s international scientific panel on climate change concluded that over a 100-year timescale the west Antarctic ice sheet was very unlikely to collapse and more likely to gain mass because of greater precipitation. The panel concluded that Antarctica was ?a slumbering giant?. Prof. Rapley said studies of the Antarctic peninsula had already shown that the collapse of floating ice shelves had acted like ?a cork pulled out of a bottle? and increased the flow of ice streams into the sea.