The UK is “woefully unprepared” for deadly heatwaves, according to a cross-party committee of MPs, with the government ignoring warnings from its official climate change adviser.

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One in five homes dangerously overheats during heatwaves today, the MPs found, while on the hottest day of 2016 alone there were almost 400 heat-related deaths. But climate change has doubled the risk of heatwaves and events even worse than the heat currently baking the nation could occur every other year by the 2040s, the MPs report said, with a tripling of heat deaths to 7,000 a year as a result.

Buildings, particularly hospitals and care homes, must be prepared for heatwaves, the MPs said, with sick and elderly people especially vulnerable to heart and breathing problems. But currently the NHS is only required to prepare plans for severe cold weather and there are no building regulations to prevent overheating with ventilation and shading.

The MPs also called for other measures, including formal guidance from Public Health England to relax office dress codes and allow home working during heatwaves, and to head teachers about relaxing school uniform policy.

The MPs said a public information campaign on the serious dangers of heatwaves was needed and health minister Steve Brine, giving evidence to the MPs in June, acknowledged this: “Heat alerts tend to be seen as barbecue alerts, as opposed to there being a risk.”

Facebook Twitter Pinterest Leominster, Herefordshire: a garden thermometer is seen reading almost 37C. Photograph: Jim Wood/Alamy

Mary Creagh MP, chair of the environmental audit committee, said: “The government has sat on its hands, ignored the warnings of the Committee on Climate Change (CCC) and left the UK woefully unprepared.” As far back as 2003, a severe heatwave saw temperatures reach 38.5C and more than 2,000 heat-related deaths in 10 days, she said: “But the government has continued to allow homes, hospitals and schools to be built that are not resilient to heatwaves.”

Kathryn Brown, head of adaptation at the CCC said: “We have recommended to government every year since 2014 that a new standard or regulation is needed to address the risk from overheating in new homes.” Ministers have rejected the advice, citing a commitment to “reduce net regulation on homebuilders”.

The government published its updated National Adaptation Strategy on 19 July but Brown said it made no commitment to deal with the issue. “Action is needed now to redouble efforts to tackle overheating – one of the key risks facing the UK from climate change.”

Heatwaves are known killers in the UK and the number of hot days is rising. “It is likely that more than 1,000 people have died in the UK as a result of the extended heatwave conditions this summer,” said Bob Ward, policy director at the Grantham Research Institute on Climate Change at the London School of Economics. “There would justifiably be an outcry if more than 1,000 people had died in a flood.”

“The government’s apparent lack of concern about the growing risks of deadly heatwaves is absolutely woeful,” he said. “The lack of communication by government departments and agencies about the risks of heatwaves is contributing to the death toll every year.”

Facebook Twitter Pinterest Penarth, Wales. A dog enjoys the shade of a bench on the esplanade. Photograph: Mark Hawkins/Alamy

The EAC report found that homes in densely populated urban areas, which can be 10C hotter than the countryside, are already at significant risk of overheating, as are certain other buildings, including houses built in the 1960s and 1970s.

“The current lack of regulation to prevent overheating, means that new developments, including hospitals and care homes, which will be around for the next 70 years will add to the number of buildings that overheat,” it said.

Excess deaths in nursing homes increased more than 40% in some parts of the UK during the 2003 heatwave, the MPs reported, but the homes are still not required to report their readiness to cope under NHS emergency preparedness rules.

Heatwaves are also economically costly, the EAC reported, particularly due to lost working days as a result of transport problems. “Only 50% of the UK’s motorways and major roads are surfaced with material that is the most resilient to the kind of summer temperatures the UK is beginning to experience regularly,” the MPs said, while in June, railway tracks buckled in the heat and caused significant delays.

Research cited in the report found that in 2010, 5m staff days were lost due to overheating above 26C, resulting in an economic loss of about £770m.

The Department for Environment, Food and Rural Affairs did not respond to a request for comment.