By 2050 heatwaves like that of 2003, which killed 15,000 in Europe and pushed British temperatures above 38C (100F) for the first time, will seem "unusually cool", the Hadley Centre for Climate Change says.
In its report Uncertainty, Risk and Dangerous Climate Change, to be published today at the climate talks in Buenos Aires, it estimates that average temperatures will rise by 3.5C, well above the 2C which the EU says is the limit to avoid catastrophic global warming.
It also says that the Greenland ice sheet could disappear, ultimately raising the global sea level by 7 metres. This could proceed at the rate of 5.5mm a year, and this with the 3mm rise caused by the thermal expansion of sea water would soon put large part of Britain, including the London docklands, under threat.
Once that process began it would be impossible to "regrow" the ice cap, the report says. The government is already concerned about the Greenland melt affecting the British climate and is spending £20m on studying it.
The fresh water being added to the Atlantic from the Arctic and Greenland ice threatens to slow or stop the current which warms the North Atlantic, the Gulf Stream.
Professor Niels Reeh of the Danish Polar Institute, who has been studying the Greenland ice for 20 years, says the loss between 1995 and 1999 was about 50 cubic kilometres of ice a year, enough to raise the global sea level by 0.13mm a year.
The Hadley Centre says that if the Gulf Stream stops warming Britain's shores, winter temperatures will plunge within 10 years, regularly reaching minus 10C.
The report says that rapid warming continued in the first nine months of 2004.
Margaret Beckett, the environment secretary, who is in Buenos Aires, hopes to boost preparation for the Kyoto protocol's coming into force on February 16. It legally binds the industrial countries which have signed it to reducing their greenhouse gas emissions.
Yesterday she stressed that Britain would use its coming presidency of the G8 and the EU to push climate change up the international agenda.
The conference will consider how to proceed after 2010, when the Kyoto agreement ends. All agree that further big cuts in CO2 emissions will be needed and that the US, India and China should come on board, but have no idea how it can be done.