The Met Office issued an amber “heat health watch warning” for parts of England this week, urging people to either stay out of the sun or avoid being in the sun when it is at its strongest between 11am and 3pm.
The amber, or level three, warning is issued when temperatures are predicted to hit 30 degrees during the day, and 15 degrees at night, for at least two consecutive days, the Met Office spokeswoman said.
There is a 90% possibility of heatwave conditions between 9am today and 9am Friday in parts of England, mainly in the south and east, she added.
The heat health watch warning is designed to make local services aware that these conditions are being met, and for them to take action.
“We advise the public to take care in the sun, especially when temperatures are potentially reaching 30 degrees or more throughout this week – either stay out of the sun or be sensible and don’t go out in the strongest sunshine hours (11am to 3pm)”, the forecaster said.
Members of the public were also urged to take the usual precautions in the sun, including covering up, wearing sun screen and drinking plenty of water.
It comes as the UK experienced its hottest day of the year so far on Monday. Temperatures soared to 33.3C in Santon Downham in Suffolk, beating the 33C recorded at Porthmadog in North Wales on 28 June. By Wednesday it could reach 34C.
The last time the mercury exceeded this was when it hit 34.5C at Heathrow on 21 June last year. The hottest July day on record is 36.7C, also recorded at Heathrow on 1 July 2015.
Forecaster Alex Burkill said the hot weather could lead to thunderstorms too.
“The heat and humidity means there is a lot of energy in the air and a high risk of intense thunderstorms. Whether or not it will happen is where the uncertainty is.”
Nearly all parts of the UK are seeing above average temperatures, but more so towards the east or south east.
During the current heatwave, UK temperatures have been approximately 10C higher than average for this time of year. Highs of around 23C can usually be expected in London in July.
Scotland, which averages 17C, could enjoy highs of up to 25C, while Wales could jump to 26C, or 27C instead of 19.2C
Those in Northern Ireland may be basking in 24C rather than the July average of 18.5C.
Meanwhile the hot weather has brought with it an unexpected - and rather unwelcome - side effect as swimmers are warned about the rising number of jellyfish off Britain’s shores.
Folkestone Rescue, a charity which helps keep people on beaches safe, urged swimmers to consider wearing full-length wetsuits and boots to protect against the possibility of jellyfish stings.
Chris Lightwing, from Folkestone Rescue, said they have had a number of reports of people being stung over the last two weeks.
“If you have been stung by a jellyfish, you should leave the water immediately and keep still. Any remaining tentacles should be removed with tweezers whilst wearing gloves.
“Taking painkillers such as paracetamol and ibuprofen will help to lessen any pain or swelling. Weak vinegar can also be used to help neutralise the sting.
“Do not apply any other substances (such as urine) to the affected area; these do not work and may make the injury worse,” Lightwing said.