London's hottest summer day, which in recent decades has averaged 30.7 degrees Celsius, or 91.6 Fahrenheit, could increase by 10 degrees C to 40.7C or 105.3F, a staggering rise – which would make travel on the London Underground, for example, where the increase would be further magnified, virtually unendurable – with a high probability of increased deaths from heat stress among the old and infirm.

Similar huge increases are forecast for every region of Britain in the first localised forecasts of the potential impacts of global warming. Also for the first time, detailed projections of drought, increased winter rainstorms and sea level rise are made for each area, showing for instance that Southwold, a Suffolk coastal resort already threatened with erosion, faces a sea level rise of 37cm, or 14in, by the 2080s, while London itself could face a similar rise, with the threat of an additional 97cm, or 3ft, of storm surge.

The long-awaited predictions, which update forecasts last made in 2002, were based on extensive computer modelling by the Met Office's Hadley Centre, and were unveiled by the Environment Secretary, Hilary Benn, who said they would "affect every aspect of our daily lives".

Despite a certain level of uncertainty in the models, Mr Benn said, it was clear that climate change was occurring, and he hit out at those denying it, following after sceptical questions in the House of Commons from two Tory MPs, Andrew Tyrie and Peter Lilley. "If there are those in society who still think this isn't happening and we don't need to worry, who think we can pull up the bedcovers and it'll all go away, they are profoundly mistaken," he said.

The predictions are based on different scenarios of the future emissions of the greenhouse gases which are causing the earth's atmosphere to warm, and thus highlight the worldwide need to cut emissions back substantially, which will be the focus of the world climate conference in Copenhagen in December.

At the moment the world is close to the medium emissions pathway, and this is likely to mean an average regional summer temperature rise by the 2080s of 4C, which could be 5C in the south-east.

Summer rainfall under this scenario is likely to drop by between 11 and 27 per cent, meaning drought, and winter rainfall is likely to increase by between 11 and 23 per cent, causing flooding.

However, the important thrust of the new projections for Britain is that we are now committed to quite a lot of climate change, no matter what we do, as the full effects of greenhouse gases take about 30 years to work through from the moment they are emitted and so there is climate change yet to come from carbon dioxide which is already up there.

So the new figures suggest it is virtually certain, whatever climate deal the world can put together, that by the 2040s, summers in southern England will be about 2.3C (more than 4F) hotter on average than they are now. This means that Britain's hottest summer in 2003 when the temperature exceeded 100F for the first time, will in 30 years' time be the norm – and by the 2080s it will seem like a cool summer.

The projections meant, said Mr Benn, "that we must plan to adapt to changes that are now unavoidable" – and they will be the starting point for a major Government effort at climate change adaptation, which will take in everything from flood defence and NHS heatwave planning to how to design roads and buildings and how to help wildlife cope with a changing world.