The impact of climate change is likely to be more severe in major cities, with the elderly most at risk, according to a study commissioned for the Greater London Authority and obtained exclusively by the BBC Ten O'Clock News.

The predicted rise in temperatures in the coming decades will be exacerbated by what scientists call the "urban heat island effect", in which temperatures during heatwaves can be 6-7C higher in cities than in surrounding areas. The new research, led by Professor Glenn McGregor of King's College London, analyses data from recent heatwaves and concludes that the risk of heat-related deaths is greater in urban areas, especially in London.

The study concludes that predictions for climate change by 2080 mean than London is likely to be hit with more cloud-free days and more frequent periods of intense heat. The researchers have devised a Heat Vulnerability Index - a highly detailed map plotting the areas of London where the highest temperatures have been recorded and where there are the highest proportions of residents who are elderly or living alone, the categories of people seen as most vulnerable.

Professor McGregor told BBC News: "What we're trying to do is identify where sensitive people live within large cities such as London, Manchester and so forth, so these people can be targeted with help during heatwave events."

Although the Met Office and Department of Health provide a heatwave alert if daytime temperatures rise above 32C, the Heat Vulnerability Index will provide far more detailed information about where the need for assistance is most acute. Professor McGregor said the experience of the heatwave of 2003, when some 30,000 people died across Europe, showed how important it was to have systems in place to cope. According to research by Britain's Hadley Centre for Climate Prediction, heatwaves of that scale are predicted to become far more common in the decades ahead.