A United Nations-sponsored panel of experts has concluded that worldwide temperatures will rise an unprecedented 2 degrees within 35 years and about 6 degrees by the end of the next century, if nothing is done to combat global warming.

The new forecast, released yesterday in Britain, predicts that temperatures will be far greater than any experienced in the last 10,000 years. The conclusions are based on the work of more than 300 climate experts from around the world, and the report is said to be the most exhaustive examination yet of the prospect for global warming. The conclusions strengthen the case of environmentalists who have been calling for dramatic reductions in output of carbon dioxide, the principal gas responsible for global warming.

Upon release of the report, Prime Minister Margaret Thatcher joined other European leaders and broke ranks with the Bush administration by pledging that Britain would eliminate growth in carbon dioxide emissions over the next 15 years -- provided other industrialized countries do the same, Washington Post correspondent Glenn Frankel reported from London.

"The problems do not lie in the future, they are here and now," Thatcher said. "And it is our children and our grandchildren who are already growing up who will be affected."

The Bush administration has resisted calls for action, stressing instead the need for more research in order to answer the lingering scientific uncertainties that surround global warming scenarios. White House officials repeatedly have said they were deferring action until the science behind the warnings was assessed by the U.N.'s Intergovernmental Panel on Climate Change, the group that made yesterday's report.

That report, combined with Thatcher's commitment to carbon dioxide controls, is expected to strengthen the hand of U.S. administration officials who favor concrete action.

The administration has been divided over the issue, with Environmental Protection Agency Administrator William K. Reilly and Secretary of State James A. Baker III pushing for more aggressive policies. But they have been challenged by White House Chief of Staff John H. Sununu and Office of Management and Budget Director Richard G. Darman, who have argued that the scientific unknowns are too great to justify large reductions in carbon dioxide and other global warming gases.

Responding to the report, Reilly said, "These are high numbers. Obviously, we'll take it very seriously."

But other administration officials were more cautious, including Michael R. Deland, chairman of the president's Council on Environmental Quality, who said, "I don't think it is the final word." White House Science Adviser D. Allan Bromley said, "I don't think it will have a major effect at all on what we are doing." Bromley continued to stress the uncertainties that surround forecasts of global warming, uncertainties that Bromley said were not addressed in the executive summary released yesterday, but are to be discussed in the more lengthy report that is still undergoing revision.

Scientists who wrote the report and the executive summary, however, said that it represented a remarkable consensus among hundreds of usually contentious scientists.

"We looked at all the model results and the historical temperatures, and this was the most likely number. This is something we feel most inclined to support," said Syukuro Manabe, a climate expert with the National Oceanic and Atmospheric Administration who was one of the report's 30 authors.

Scientists predict that the planet will warm because gases such as carbon dioxide are increasing in the atmosphere, where they act as a blanket to trap heat from the sun and warm the Earth. Carbon dioxide is the ubiquitous byproduct of the industrialized age, produced by nearly all forms of burning, including combustion of gasoline, coal and oil, and by the destruction of forests.

In their report, the researchers said global temperatures would increase by about a half a degree per decade on average, resulting in an average global increase of 2 degrees by 2025 and 6 degrees by the end of the 21st century.

The scientists cautioned that land surfaces would warm more rapidly than the oceans, and that temperatures in southern Europe and central North America, for example, are predicted to be higher than the global averages. These regions would also experience reduced summer rains and drier soils, which could parch agricultural lands.

A hotter world would also cause the water in the oceans to expand, raising global sea level by about 8 inches by 2030.

While the scientists did not recommend actions to combat warming, they concluded that long-lived gases, such as carbon dioxide and other greenhouse gases -- including methane and chlorofluorocarbons (CFCs) -- would have to be reduced immediately by 60 percent to stabilize concentrations at today's levels.

In her speech yesterday, Thatcher said Britain is prepared "to set itself the very demanding target" of a 30 percent reduction in the currently projected levels of future carbon dioxide emissions. In effect, this would mean returning emissions in Britain to their 1990 level by 2005.

Thatcher's pledge brings her closer to the position of other European governments. West German officials are considering cutting carbon dioxide 25 percent by 2005. The Dutch are planning to cut emissions 8 percent by the year 2000.

"What's important about Thatcher's statement is that she's accepted the scientific evidence about what's causing the problem and accepts that it is a really serious problem that requires policy measures," said Rafe Pomerance of the World Resources Institute in Washington, one of the groups lobbying for tougher emissions standards. "That's very different from the U.S. position."

A spokesman for the prime minister's office refused to comment on the differences between Thatcher and President Bush on global warming. "It's not for us to make that judgment," he said. "What she's doing is responding to the report, which shows accumulating evidence of damage to the global environment."

Pomerance said the new report could prove crucial in persuading governments to endorse tougher standards. "You have the best scientists in the world representing governments telling policy-makers that the greenhouse effect is going to happen, the Earth is going to warm and warm significantly," he said. "It gives those governments a firm scientific basis on which to make policy. It's a culmination of 10 years of consensus-building on the science."