Illnesses caused by cold housing cost the NHS £145 million a year, according to new figures.
Poor insulation, faulty boilers and unaffordable heating costs are leading to conditions including respiratory illness, stroke and hypothermia, they suggest.
The data - for privately rented homes - was compiled by public health experts at the Chartered Institute for Environmental Health.
It was launched by Friends of the Earth as part of a new campaign involving more than 35 organisations to change the law to protect tenants from cold housing.
The data for England found that 655,800 homes rented from a landlord or letting agency are so cold they are officially a health hazard, with a bottom-of-the-scale energy efficiency rating of F or G.
Friends of the Earth together with charities including Crisis, Citizens Advice and Age UK, are calling for legislation in the Government’s new Energy Bill to make it an offence for landlords to re-let cold homes until they have been improved.
The Chartered Institute for Environmental Health’s head of policy, David Kidney, said: “The £145 million that the coldest rented homes cost the NHS each year is just the tip of the iceberg.
“Our research shows homes rented from a landlord or letting agency are more likely to be poorly insulated and cold - we need Government action to make sure all rented properties meet a decent standard of energy efficiency.
“The new Energy Bill should give local authorities powers to step in to ensure landlords improve the worst homes - and tenants who file environmental health complaints need legal protection from being evicted.”