Thunderstorms are predicted across the Portsmouth area today.
Strong and violent storms will mark the end of a three-week heatwave which has seen the hottest temperatures in seven years, weather experts have said.
Thunder, lightning and hail began overnight and will last intermittently throughout today and tomorrow, coupled with torrential rain and the risk of flash floods.
The sharp change in weather comes after weeks of scorching temperatures, which yesterday peaked at 33.5C in London - the hottest day since July 20, 2006.
The storms will thunder over the south as well as eastern, central and northern parts of England, with the East Midlands, London and Kent seeing the worst of the weather.
Temperatures will dip to the high 20s and remain very humid in the South East, but the west of England, Wales and Northern Ireland will be far cooler before the rest of the country gradually follows.
Forecaster Brendan Jones of MeteoGroup, the weather division of the Press Association, said: “In the last few weeks an area of high pressure has hung over the UK creating settled, very hot conditions.
“But now air is pushing in from the Atlantic and meeting this humid air, and the combination is causing these strong and violent storms that will last on and off throughout the day and linger into Wednesday.
“The main threat is of flash flooding from localised but torrential rain, especially as the ground is so dry and solid it will not soak up the water very easily.
“Another threat is from the frequency of the lightning bolts, which could come down to the ground and cause damage.
“It will remain quite hot and humid in some areas today, but generally everywhere will now start to cool off.”
He added that humid conditions will bring big hailstones which have already been seen in south London.
The Met Office issued a low-level alert warning of storms and torrential downpours across England, Wales and Scotland.
It said: “The public should be aware of the risk of localised disruption to travel, and more generally to summer holiday activities, due to, for example, surface water flooding.”
The overall trend for the rest of the week will see the UK gradually cool down, with a continuing risk of showers and thunderstorms.
By next week the temperatures are expected to settle in the low 20s.
The hot weather has taken its toll on the UK in recent weeks, with grass fires in London, mountain blazes in the Welsh valleys and forest fires in Fife, Scotland. Hundreds of premature deaths are believed to have been caused by the heatwave.
Wildlife experts are warning of the risk of fires in important landscapes that are in “tinder box” condition after days of hot, dry weather.
Devon Wildlife Trust has teamed up with Devon & Somerset Fire & Rescue Service to urge people to take care to avoid fires in the region, which can devastate habitats that are key to rare species as well pose dangers to people and farmland.