Health officials are warning people to take care as the UK continues to bake in a prolonged heatwave.
People should be drinking plenty of water, wear loose-fitting clothes and avoid the midday sun, according to Dr Richard Jarvis from Public Health England.
The Met Office issued a level 3 alert for London and the south east on Wednesday, which was officially the hottest day of the year.
While the warm weather has been welcomed by sunbathers, sport watchers and barbecue fans, health officials said the heat could be dangerous for very young children, elderly people, pregnant women and those with serious illnesses.
The heat has also caused problems for drivers and rail passengers in parts of England this week, after road surfaces melted and tracks buckled in the heat.
In other developments:
Police in Norfolk are warning of the dangers of swimming in open water. It comes after the bodies of a man and a teenage boy were recovered from a water-filled quarry near King's Lynn, Norfolk
Police in Newcastle say lives are being put at risk by vandals breaking fire hydrants and spraying themselves with water to cool down
The heatwave has brought a 15% rise in the demand for tap water in London and the Thames Valley. Thames Water's nine million customers have been using about 400m litres a day in addition to the 2.6bn litres they usually get through
Water UK, which represents all major water companies, says there are no plans to impose a hosepipe ban as reservoir levels are where they should be
Firefighters in London have dealt with twice as many grass fires compared with last year. There have been 1,010 incidents so far this summer
Doctors in Wales say the number of people suffering sunstroke, sunburn and heatwave-related injuries is stretching hospital emergency departments
Some MPs want bosses to send staff home if workplaces get hotter than 30C. The proposal is contained in a parliamentary early-day motion. There is no suggested maximum limit in the current rules
Level 3 is one notch below the most serious warning in the Met Office's heat-health watch system.
The temperatures that trigger local action plans vary according to location and may involve health, housing, and social care services, to protect those susceptible to the heat.
In north-east England it is 28C, in Wales 30C and in London 32C. The night-time trigger temperatures vary from 15C to 18C.
Health officials in England are advising people experiencing the very hot weather to stay cool, drink lots of cold fluids and keep an eye on those they know to be at risk.
Dr Angie Bone, who is leading Public Health England's heatwave plan, said its efforts involved health and social care workers in the community; hospitals and care homes regularly checking on vulnerable patients; sharing sun safety messages; and making sure room temperatures were set below 26C.
Under the plan, officials ensured patients had access to cold water and ice, and that medicines were stored in a cool place, she added.
The NHS says the main risks posed by a heatwave are dehydration, overheating, heat exhaustion and heatstroke.