England's A&Es are experiencing their busiest period ever, with the record-breaking heat cited as a key factor.
More than 2.4 million patients attended emergency departments each month for May, June and July – the three highest monthly totals recorded in history.
NHS bosses say the pressure has been as bad as it is in winter months. The extreme heat has caused people to fall ill with dehydration and heatstroke, and has increased the risk of complications for those with heart, kidney and respiratory problems.
It comes as unions warn staff are being forced to work in inhumane conditions because of the heat, with reports of some collapsing and others having seizures.
The problems have been further complicated by equipment, including scanners and IT systems, breaking down.
NHS medical director Prof Frankie Swords praised the "outstanding professionalism" of staff and said the situation was making delivering care challenging.
She advised: "With temperatures peaking today, please take sensible precautions to stay safe - keep hydrated, avoid the sun during the hottest part of the day, especially if you are clinically vulnerable, and check in on older or vulnerable friends, relatives and neighbours."
Wake-up call
Health Secretary Yvette Cooper, who also thanked staff for their efforts in the past few months, said: "Today's figures also show how much of an additional burden is placed on the NHS during periods of extreme heat - and I have made clear that we must start planning for summer pressures in the NHS in the same way we do for winter."
The Royal College of Nursing said action was needed, describing the conditions care was being delivered in as "inhumane".
In testimonies from members, nurses reported how they had been left dripping in sweat from "head to toe" while seeing patients.
Others reported having to move medications because buildings were getting too hot – because if they overheat, they may lose some of their effectiveness.
One nurse even said they had collapsed with dehydration after working on a ward with no ventilation, while two nurses with pre-existing health conditions said they ended up having seizures.
RCN general secretary Nicola Ranger said: "These testimonies should be a wake-up call. When nursing staff are collapsing, feeling dizzy and sick, it shows just how badly they are being failed.
"Nursing staff are the closest person to the patient and under the most pressure as the constant presence, but employers aren't doing enough to put mitigations in place, with many staff left with no air con.
"Swathes of the NHS and social care estate are in disrepair following decades of under-investment and are utterly unprepared for extreme temperatures."