The hottest day on record in the UK, on 25 July this year, caused the deaths of more than 200 additional people than usual, according to official figures that underline the deadly impact of the climate crisis.

The Office for National Statistics (ONS) said more people died on 25 July than any other day in the three months from July to September this year as the thermometer hit a record-breaking 38.7C (102F). On a normal day in July, there are about 1,150-1,200 recorded deaths in the UK, but when the temperature spiked, the rate rose to 1,404.

There were also higher-than-average deaths the day after the record-breaking temperature, with 1,331 people dying on 26 July. In total, the heatwave, although relatively brief compared with others in recent years, is likely to have caused more than 900 deaths, according to academics.

Bob Ward of the Grantham Research Institute on Climate Change and the Environment at the London School of Economics said: “These figures probably understate the full picture. We expect the final figures from Public Health England in January to show that even more people died.”

The scorching temperatures that also broke records across Europe were particularly distressing for vulnerable older people, especially those with pre-existing respiratory or cerebrovascular diseases.

Ward said the data indicated deaths started to rise every day the temperature went above 28C. “Every day that temperatures are above 28C is a threat to people with respiratory diseases,” he said.

The ONS did not directly attribute the excess death rate to the climate emergency, although it said there was a clear link between extreme heat and the number of deaths.

“When comparing the number of deaths per day in 2018 with the five-year average, we can see that some days have more deaths than we would expect. The comparatively high number of deaths occur mainly on days that are defined as heatwaves by Public Health England,” it said.

The ONS added that after a rise in deaths, there was a period when deaths tended to be lower than the five-year average. “Extreme heat seems to have an impact on the number of deaths, but across the summer period as a whole, the number of deaths is similar to previous years,” it said.

Ward said more action needed to be taken to prevent heat-related deaths, particularly in housing design. “The Committee on Climate Change reported in July this year that we have a real problem of homes that are not well adapted to heat, in particular care homes,” he said.

“It is clear that the incidence of heatwaves is increasing, both in frequency and intensity, directly as a result of climate change. But we are still building homes that are not taking into account global heating.”

Air conditioning was not the answer, he said. “All air conditioning does is dump the heat out on the streets, it doesn’t get rid of it. And most people can’t afford to install and run air conditioning. It’s not a solution, instead it makes it worse and worse. But there are simple things people can do – such as keeping curtains shut during the height of the sunshine, and opening them in the evening,” Ward said.

Last year, a cross-party committee of MPs said the UK was “woefully unprepared” for deadly heatwaves. The MPs said the government had ignored warnings from its official climate change adviser, and that without action, heat-related deaths would triple to 7,000 a year by the 2040s.