The summer heatwave could bring its most intense conditions yet today, with soaring temperatures potentially accompanied by thunderstorms.
Motorists have been warned that lightning, flash flooding and large hailstones could hit eastern parts this afternoon, with a thunderstorm warning in place from about 2pm until just before midnight.
Temperatures are forecast to be in the mid-30s for much of east and south east England, reaching 37C in London, and could rise higher still if there is a lot of sunshine, Met Office meteorologist Alex Deakin said.
In the North East, temperatures are set to peak at about 25C - and the thunderstorm warning applies here.
There is the potential for thunderstorms to develop during this afternoon, lasting well into the evening and perhaps developing into a larger area of rain.
Where the thunderstorms occur, torrential downpours are possible, with as much as 30mm of rainfall in an hour and 60mm in three hours.
Large hail and strong, gusty winds will be additional hazards. However, many places will see much lower rainfall amounts.
Forecasters say flooding of homes and businesses could happen quickly, with damage to some buildings from floodwater, lightning strikes, hail or strong winds
Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services
Spray and sudden flooding could lead to difficult driving conditions and some road closures
Power cuts might occur and other services to some homes and businesses could be lost
Mr Deakin said: "The reason there's some uncertainty is because of the thunderstorms, how many we see and where they get going ... torrential rain, the risk of local flooding, large hailstones potentially damaging, lots of flashes of lighting and rumbles of thunder.
"Roads like the M11, M18, M1 and A1 won't be very pleasant, a lot of spray and surface water around."
Yesterday saw the hottest day of the year so far, with temperatures of 35C (95F) recorded at Heathrow in London.
A mix of toxic air, extreme highs, emissions from the continent and a lack of cloud cover caused a "high" air pollution alert to be issued for London.
Meanwhile authorities said the heatwave was causing "winter conditions" in parts of the NHS, while many nurses are said to be dizzy and exhausted.
Fire brigades also called for a ban on barbeques in parks, and drivers were urged not to throw rubbish following a string of grassland fires in recent weeks.
Going into the weekend, most places will see temperatures closer to average for the time of year, around the high teens to low 20s, with the chance of showers and strong breezes, Mr Deakin said.
Some areas will see a more than 10C drop between Friday and Saturday, he added.
He went on: "Weather fronts are trying to come in from the Atlantic and eventually they will do so.
"And that has the impact of ousting the really hot and humid air .. . it turns runs cooler for the weekend, fresher, more comfortable at night..."