British scientists have discovered a threat to the world that may be a result of global warming.

An international conference on climate change has been told that a massive Antarctic ice sheet, whose collapse would raise sea levels around the world by more than 5m, and which has been assumed to be stable, might be starting to disintegrate.

Researchers from the Cambridge-based British Antarctic Survey are measuring remote points in the West Antarctic Ice Sheet where they have recently found ice to be flowing out into the sea at the enormous rate of 250cu km a year.

This discharge alone is raising global sea levels by a fifth of a millimetre a year.

Professor Chris Rapley, the survey's director, told the conference at the UK Meteorological Office in Exeter that their discovery had reactivated worries about the ice sheet's collapse - which only four years ago were firmly dismissed.

He said: "The last report [by the UN's Intergovernmental Panel on Climate Change] characterised Antarctica as a slumbering giant in terms of climate change.

"I would say that it is now an awakened giant. There is real concern."

He added: "The previous view was that the West Antarctic Ice Sheet would not collapse before the year 2100. We now have to revise that judgment. We cannot be so sanguine."

Collapse of the sheet would be a disaster for the whole world, putting enormous swathes of low-lying, desperately poor countries such as Bangladesh under water.

The conference has been called by British Prime Minister Tony Blair as part of Britain's efforts to increase the pace of international action on climate change.

Blair has asked it to explore the question of just how much climate change the world can take, before the consequences are catastrophic for human society and natural ecosystems.

It heard several alarming warnings of possible climate-related catastrophic events, including the failure of the Gulf Stream, which keeps the British Isles warm, and the melting of the ice sheet covering Greenland.

But it was the revelations of Rapley, head of one of the world's most respected scientific bodies, which were the most dramatic, as they reopened a concern which many scientists assumed had been laid to rest.

Antarctica as a whole is land covered by very thick ice, but the ice sheet covering the eastern half of the continent is very stable as it sits on rocks which are well above sea level.

Worries about the ice covering the western half first surfaced more than 25 years ago when it was realised that there, the base rocks are actually well below the level of the sea.

In some circumstances, it was feared, such as a melting of the edge of the ice sheet because of rising temperatures, sea water could get under it and eventually lead to its collapse.

The dramatic discharge into the Amundsen Sea completely opened up the whole debate, Rapley said.

It had only been recently discovered because the area was so remote. But British scientists, with United States logistical help, had now established a base in the area to investigate further.