By David Shukman
Environment correspondent, BBC News
Campaigners tried to point up the urgency of the UN climate talks
In a dramatic acceleration of forecasts for global warming, UK scientists say the global average temperature could rise by 4C (7.2F) as early as 2060.
The Met Office study used projections of fossil fuel use that reflect the trend seen over the last 20 years.
Their computer models also factored in new findings on how carbon dioxide is absorbed by the oceans and forests.
The finding was presented at an Oxford University conference exploring the implications of a 4C rise.
The results show a "best estimate" that 4C (measured from pre-industrial times) will be reached by 2070, with a possibility that it will come as early as 2060.
Richard Betts of the Met Office Hadley Centre described himself as "shocked" that so much warming could occur within the lifetimes of people alive today.
"If greenhouse gas emissions are not cut soon then we could see major climate changes within our own lifetimes," he said.
"Four degrees of warming averaged over the globe translates into even greater warming in many regions, along with major changes in rainfall."
The model finds wide variations, with the Arctic possibly seeing a rise of up to 15C (27F) by the end of the century.
Western and southern parts of Africa could warm by up to 10C, with other land areas seeing a rise of 7C or more.
In its 2007 assessment, the Intergovernmental Panel on Climate Change (IPCC) said the average warming by the end of the century would probably lie between 1.8C and 4C (3.2-7.2F), though it did not rule out the possibility of larger rises.
Key to the Met Office calculations was the use of projections showing fossil fuel use continuing to increase as it has done for the last couple of decades.
"Previously we haven't looked at the impact of burning fossil fuels so intensely," said Dr Betts.
"But it's quite plausible we could get a rise of 4C by 2070 or even 2060."
Dr Betts and his colleagues emphasise the uncertainties inherent in the modelling, particularly the role of the carbon cycle.
But he said he was confident the findings were significant and would serve as a useful guide to policymakers.
The presentation at Oxford's Environmental Change Institute came as negotiators from 192 countries were gathering in Bangkok for the latest set of prepatory talks in the run-up to December's UN climate summit.
Major governments of developing and industrialised nations are committed to a deal that would keep the global temperature rise to 2C, which many regard as a threshold for "dangerous" climate change.