The heatwave is being blamed for record numbers of people going to A&E in England in July.
As temperatures soared, attendances reached more than 2.1 million – the highest since records began in 2010.
The number of patients waiting more than a year for non-urgent surgery also rose to their highest point in more than six years, at 3,517.
This is despite a pledge in 2014 by the then Health Secretary Jeremy Hunt to end the “unacceptable” waits.
The figures are part of a raft of data released on Thursday by NHS England.
Health bodies said the heatwave had added to existing pressures on the NHS, with reports of more people turning up with dehydration, heart failure and kidney problems related to the unusually high temperatures.
Prof John Appleby, chief economist of the Nuffield Trust health think tank, said: “July 2018 was the most pressured summer month for A&E departments in recent history, showing that there’s no doubt this summer’s heatwave has caused severe strain on the NHS.”
A&E attendances reached 2,176,022 in July – up from 2,091,318 in June – although attendances have steadily climbed since 2010.
The Society for Acute Medicine said the summer was usually a time for frontline staff to “recharge the batteries” but that had not happened this year.
Dr Nick Scriven, the society’s president, said: “Last year NHS leaders admitted it took until October to recover from winter 2017 and we are now only a few months away from the next onslaught…
“On the plus side, this may have triggered alarm bells earlier than normal and lead to some positive actions being put in place to help us contend with the inevitable winter pressures we will face.”
The Royal College of Nursing said the heatwave had placed winter-like strains on the health service.
Donna Kinnair, director of nursing policy and practice at the RCN, said nurses were seeing more cases of heart failure, renal failure and dehydration, with older people being particularly at risk.
An NHS England spokesman added that there had been an “unprecedented summer surge” in July.
“Thanks to the hard work of staff, nine in 10 people were seen, treated and admitted or discharged within four hours,” the spokesman said.
Separately, a survey of 760,000 people across England on their views of their GP practices showed nearly 40% of patients were unable to get an appointment at the time they wanted.
It also found that just 66% of patients who wanted a same-day appointment said they managed to get one.
However, nearly 84% described their overall experience of their surgery as very or fairly good.
And 68.6% rated their overall experience of making an appointment as good.
About a third (35.6%) of patients who wanted to see a GP but found their practice was closed said they ended up going to A&E.
The survey of patients in England has been redesigned and now includes 16 to 17-year-olds for the first time, so results cannot be compared to previous years, NHS England said.