Experts have warned people not to exercise in high temperatures due to “deadly conditions” caused by the heat.
Body heat during exercise combined with the temperatures could see even the fittest people suffering heat exhaustion, they said.
The UK’s hottest day on record is predicted for Tuesday with highs of up to 42C.
Heat can also trigger mental health issues, according to Dr Lawrence Wright.
Climate scientist at the University of Bristol Cabot Institute for the Environment, Dr Wright said: “Everyone is at risk and we do need to be aware and take precautions and definitely not view these as normal summer days or something to go out and have fun in.”
Mike Tipton, professor of human and applied physiology at the University of Portsmouth, said: “We produce a lot of heat when we exercise.
“And we’re now at a temperature where … actually people will warm up, just doing their day-to-day activities in the house, or outside, and cooking, that sort of thing.
The “unprecedented temperatures” mean people should avoid exercising outside for the next couple of days, they added.