Thousands flocked to beaches amid warnings high temperatures over the next few days could result in people dying.
Tracy Nicholls, chief executive at the College of Paramedics, said the “ferocious heat” predicted to hit the UK could have a detrimental effect on Britons.
But Deputy Prime Minister Dominic Raab said people should be resilient enough to “enjoy the sunshine”.
Scorching temperatures are predicted for Monday, with Peterborough expected to hit 37C and Milton Keynes, Norwich and Lincoln set to see 36C – while temperatures could rise to 40C in London on Tuesday.
Climate attribution scientist at the Met Office, Dr Nikos Christidis, said the 40C prediction is a result of climate change.
The forecaster issued an amber warning for the majority of England on Sunday. The alert extends to southern Scotland and Wales from Monday until Tuesday.
The UK’s first red extreme heat warning has also been issued by the Met Office across a large part of England from London to Manchester and York on Monday and Tuesday.
The UK Health Security Agency has also issued a heat health warning at level four, which is described as a “national emergency”.
Ms Nicholls told Sky News’s Sophy Ridge On Sunday programme: “This isn’t like a lovely hot day where we can put a bit of sunscreen on, go out and enjoy a swim and a meal outside.
“This is serious heat that could actually, ultimately, end in people’s deaths because it is so ferocious. We’re just not set up for that sort of heat in this country.”
Her comments were made after Mr Raab said people should take precautions ahead of the record-breaking temperatures but added that they should be able to enjoy themselves.
“Obviously there is some common-sense practical advice we are talking about – stay hydrated, stay out of the sun at the hottest times, wear sun cream – those sorts of things,” he told Sophy Ridge On Sunday.
“We ought to enjoy the sunshine and actually we ought to be resilient enough through some of the pressures it will place.”
Ministers held a virtual emergency Cobra meeting on Saturday after meteorologists warned the record high temperatures could put lives at risk.
Additional contingency support for ambulance services, such as more call handlers and extra working hours, have been put in place on Monday and Tuesday.
Transport services are expected to be disrupted on both days, with Cabinet Office Minister Kit Malthouse urging people not to travel.
Avanti West Coast, Transport for London and London North Eastern Railway are also urging passengers not to travel unless it is absolutely necessary.
Meanwhile, the chairman of the NHS Confederation said hospitals are going to be “really, really pushed” over the next few days.
Lord Victor Adebowale said the NHS “will cope”, but added that “coping isn’t good enough”.
He told Times Radio: “My members are pretty stretched at the moment. Ambulances are operating at their peak, the waiting times for ambulances are now getting longer.
“The NHS will cope but coping isn’t good enough. We need to be actually operating in a way that allows people to get the treatment they need wherever needed, in good time. And that’s a struggle.”
Greater Manchester Police implored people to avoid cooling off in reservoirs, rivers or ponds after a teenage boy died while swimming with friends in a canal.
The 16-year-old got into difficulties and was last seen struggling in the water at Salford Quays in Greater Manchester at around 6.15pm on Saturday.
A boy’s body was recovered overnight and officers believe it may be that of the missing teenager.
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