The Met Office has issued yellow thunderstorm warnings for the next five days, with the potential of flash flooding or damage to buildings from lightning and hail.
Warnings of further unsettled weather follow stormy scenes across parts of the country since Tuesday, amid sweltering conditions and “tropical nights”, where after-dark temperatures do not fall below 20°C (68°F).
Met Office forecaster Matthew Box said: “We’ve got the risk of thunderstorms right through to the start of next week, and maybe even beyond that.”
A yellow thunderstorm warning covering all of Wales, southern England and areas in the North West and Midlands is in place until midnight on Thursday.
The Met Office warns that while some areas could stay dry, others could see severe thunderstorms and flash flooding, with as much as 40mm of rain falling in less than an hour.
There were thundery showers across southern England on Thursday afternoon, with a collection of thunderstorms across the south-west.
Devon and Somerset Fire and Rescue Service said on Twitter it had received numerous calls relating to flooding in properties in Devon, with some residents reporting up to 18 inches of water inside their properties.
Some six flood alerts, meaning flooding is possible, have been issued by the Environment Agency for parts of the West Midlands, including the River Blythe in Warwickshire.
Drivers were caught by flash flooding following “biblical rain and hail” on the M25, as one onlooker said that more than 20 cars looked as if they were stranded and that the water was feet-high in some areas.
One road user tweeted: “M25 flooded near Junction 7. Biblical rain and hail. Clockwise traffic at a standstill.”
Temperatures were cooler compared to the rest of the week, with the mercury reaching 29.6°C (85.3°F) in the village of Frittenden in Kent, according to forecaster Mr Box.
It comes after temperatures above 34°C (93.2°F) were recorded for the sixth day in a row in parts of southern England on Wednesday, for the first time since at least 1961.
Severe storms hit parts of the UK on Wednesday evening, with fire services called to deal with incidents of flash flooding and disruption to roads, while a bolt of lightning struck an area in Wrexham, Wales.
Overnight into Friday, there is a further risk of storms across the south, particularly in the West Country and South Wales, while conditions are likely to remain humid.
“Although we’re unlikely to see overnight temperatures stay above 20°C (68°F), apart from maybe in London, most places will see temperatures in the high teens for the majority of the UK, particularly in the south,” Mr Box said.
The south-west and parts of South Wales could see more thundery showers on Friday, while it will be a cloudy start for many, particularly across central and eastern England.
A yellow thunderstorm warning issued for Friday covering Wales and much of England has been extended until midday on Saturday, with further thunderstorms expected.
Towards the west, including Northern Ireland, north-west England and western Scotland, Friday is likely to be another day of sunshine, according to Mr Box.