A critical meltdown of ice sheets and severe sea level rise could be inevitable because of global warming, the world's scientists are preparing to warn their governments. New studies of Greenland and Antarctica have forced a UN expert panel to conclude there is a 50% chance that widespread ice sheet loss "may no longer be avoided" because of greenhouse gases in the atmosphere.

Such melting would raise sea levels by four to six metres, the scientists say. It would cause "major changes in coastline and inundation of low-lying areas" and require "costly and challenging" efforts to move millions of people and infrastructure from vulnerable areas. The previous official line, issued in 2001, was that the chance of such an event was "not well known, but probably very low".

The melting process could take centuries, but increased warming caused by a failure to cut emissions would accelerate the ice sheets' demise, and give nations less time to adapt to the consequences. Areas such as the Maldives would be swamped and low-lying countries such as the Netherlands and Bangladesh, as well as coastal cities including London, New York and Tokyo, would face critical flooding.

The warning appears in a report from the Intergovernmental Panel on Climate Change, which assesses the likely impacts of global warming and will be published in April. A final draft of the report's summary-for-policymakers chapter, obtained by the Guardian, says: "Very large sea level rises that would result from widespread deglaciation of Greenland and West Antarctic ice sheets imply major changes in coastlines and inundation of low-lying areas, with greatest effects in river deltas.

"Relocating populations, economic activity and infrastructure would be costly and challenging. There is medium confidence that both ice sheets would be committed to partial deglaciation for a global average temperature increase greater than 1-2C, causing sea level rise of 4-6m over centuries to millennia." Medium confidence means about a five in 10 chance.

The revelation comes as a new report points out that greenhouse gas emissions running into hundreds of millions of tonnes have not been disclosed by Britain's biggest businesses, masking the full extent of the UK's contribution to global warming. According to a report by Christian Aid, only 16 of Britain's top 100 listed companies are meeting the government's most elementary reporting guidelines on greenhouse gas emissions. As a result, almost 200m tonnes of damaging CO2 is estimated to be missing from the annual reports of FTSE 100 companies. The figure is more than the annual reported emissions of Pakistan and Greece combined.

This month the IPCC published a separate study on the science of climate change, which concluded that humans are "very likely" to be responsible for most of the recent warming, and that average temperatures would probably increase by 4C this century if emissions continue to rise. Even under its most optimistic scenario, based on a declining world population and a rapid switch to clean technology, temperatures are still likely to rise by 1.8C.

The new report is expected to say this means there is "a significant probability that some large-scale events (eg deglaciation of major ice sheets) may no longer be avoided due to historical greenhouse gas emissions and the inertia of the climate system". Scientists involved with the IPCC process cannot talk publicly about its contents before publication. But a senior author on the report said: "It's not rocket science to realise that with the numbers coming out from the IPCC [science report], the warming by the end of the century is enough to do that." The report's conclusion poses a conundrum for governments of how to address a problem that is inevitable but may not occur for hundreds or thousands of years. "That's for the policy makers to decide but it really is a very difficult question," the source said. "Those are moral questions and the answer you give will depend very much on which part of the world you live in."

Jonathan Overpeck, a climate scientist at the University of Arizona, said the key question was not whether the ice sheets would break up, but how quickly. Some models suggest rapid melting that would bring sea level rises of more than a metre per century. "That would be much harder for us to cope with," he says.

The IPCC science report predicted sea level rises of up to 0.59m by the end of the century. But that does not include the possible contribution from ice sheets, because the experts judged it too unpredictable to forecast over short timescales.