The global sea level looks set to rise far higher than forecast because of changes in the polar ice-sheets.
Scientists at a climate change summit in Copenhagen said earlier UN estimates were too low and that sea levels could rise by a metre or more by 2100.
The projections did not include the potential impact of polar melting and ice breaking off, they added.
Ten per cent of the world's population - about 600 million people - live in low-lying areas.
The UN's Intergovernmental Panel on Climate Change (IPCC), in its 2007 Fourth Assessment Report, had said that the maximum rise in sea level would be in the region of 59cm.
But Professor Konrad Steffen from the University of Colorado, highlighted new studies into ice loss in Greenland, showing it has accelerated over the last decade.
The forecasts by the team of scientists are critically important for coastal communities.
