An amber "heat health watch" warning has been issued for parts of England this week as the UK braces itself for the hottest day of the year so far.
The amber, or level three, warning is issued when temperatures are predicted to hit 30C (86F) during the day, and 15C (59F) at night, for at least two consecutive days, the Met Office spokeswoman said.
People were urged to either stay out of the sun or at least avoid being in the sun when it is at its strongest between 11am and 3pm.
Forecasters said there was a chance of hitting 34C or 35C (95F), which could put Britons in line for the hottest day since the mercury hit 34.5C (94.1F) at Heathrow on June 21 last year.
The hottest July day on record is 36.7C (98F), which was reached at Heathrow on July 1 2015.
In Sheffield, temperatures are expected to reach 28C today before cooling on Tuesday and Wedesday.
The temperature is then expected to reach 27C on both Thursday and Friday before again cooling to 23C on Saturday and Sunday.
Porthmadog in North Wales holds the record for the hottest day of the year so far, with temperatures reaching 33C (91.4F) on June 28.
There is a 90% possibility of heatwave conditions between 9am on Monday and 9am Friday in parts of England, mainly in the south and east, the Met Office said.
The heat health watch warning is designed to make local services aware that these conditions are being met, and for them to take action.
Nearly all parts of the UK are seeing above average temperatures, but it will be hot or very hot towards the east or south east.
During the current heatwave, UK temperatures have been approximately 10C higher than average for this time of year.
Instead of 23C (73.4F) which could usually be expected in London in July, temperatures could tip 34C (93.2F).
Scotland, which averages 17C (62.6), could enjoy highs of up to 25C (77F), while Wales could jump to 26C (78.8F) or 27C (80.6F) instead of 19.2C (66.5F).
Those in Northern Ireland may be basking in 24C (75.2F) rather than the July average of 18.5C (65.3F).
"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 Met Office spokeswoman 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.