The threat to Britain posed by floods, heat waves and coastal erosion is far more serious than previously thought, according to a group of experts appointed by the Government.

Their report, to be published today, is expected to be the bleakest official assessment yet of the impact of climate change in Britain over the rest of this century.

It will say that the estimated number of homes at risk of flooding is likely to double to about 800,000 within 25 years because of rising sea levels. Average summer temperatures in the South of England will rise by 2C by the 2040s and up to 6.4C by 2080, it will warn, increasing the risk of skin cancers and insect-borne diseases.

Hilary Benn, the Environment Secretary, will respond to the warning from the UK Climate Impact Programme (UKCIP) by urging people and businesses to redouble efforts to reduce carbon emissions. Contingency measures will be announced to prevent hospitals becoming overwhelmed during prolonged periods of hot weather. Mr Benn is also likely to signal the urgent need to extend water metering and change agriculture practices as he lays out a “five-point” plan.

More than 100 public sector organisations — including the NHS, police and Army — will be ordered to develop and publish action plans by next year on how they intend to respond to the dangers.

The increased threat of flooding to low-lying areas such as Norfolk and of river flooding in towns and cities such as Sheffield and Tewkesbury, Gloucestershire, will make more urgent the need to bolster flood defences. There is, however, no significant new money for schemes to protect against the increased threat of flooding and coastal erosion. Mr Benn is expected instead merely to outline the work that is already taking place when he announces the climate change projections in the House of Commons today.

The UKCIP study is funded by the Department for Environment, Food and Rural Affairs and includes scientists from the Met Office and the University of Oxford. It will give detailed projections for this century for Britain using a variety of emission scenarios. It will also have projections of sea-level rises and storm surge frequency around the coast.

The research updates a 2002 study, that concluded that Britain would continue to get warmer — with hotter and drier summers and milder and wetter winters — and that sea levels would continue to rise.

Andy Atkins, of Friends of the Earth, said that the report would demonstrate the need for an urgent response to climate change. “The UK Government must take tougher action and show real leadership by example ahead of crucial climate negotiations in Copenhagen in December.”

Robin Oakley, of Greenpeace, said that the findings would show that millions more homes in Britain would be at risk of flooding. “These projections are extremely scary,” he said. “The UK really needs to accelerate its level of action on slashing emissions.”

Story by Francis Elliott, Deputy Political Editor, and Robin Pagnamenta, Energy Editor in the Times