Thunderstorms are moving across parts of the UK, after some areas saw the longest stretch of high temperatures since the 1960s.
The severe weather caused flash floods in parts of southern England on Thursday, bringing travel disruption.
Yellow storm warnings apply to much of England, Wales, parts of Northern Ireland and south-west Scotland following highs of more than 30C.
It comes after torrential rain and lightning lashed parts of Scotland.
The yellow warning applies to Wales and the majority of England until Monday night next week, and forecasters have warned of potential damage to buildings from lightning strikes or strong winds.
There is also a small chance of flooding, travel disruption and power cuts in those regions, the Met Office said.
As of 15:00 BST, the Met Office said thunderstorms had broken out across the southern counties of England between Devon and Kent, with frequent lightning, heavy downpours, large hail and gusty winds possible in the coming hours.
Earlier, it reported thunderstorms had moved across the southern Midlands, East Anglia and north-west London.
BBC Weather said about 8,000 lightning strikes were detected across parts of southern England in the hours leading up to 17:00 BST, as the severe thunderstorms “slowly rumble their way westwards”.
Its forecasters predicted highs of 29C in the south-east of England on Thursday, and cooler temperatures in Scotland and the north-east of England.
Heavy downpours on Thursday afternoon sparked travel disruption on rail and roads in southern England.
There are closures in both directions on the M25 between junctions six and nine in Surrey due to flooding, according to Highways England. The M23 southbound is also closed between junctions seven and eight.
Motorists have been urged to allow additional time for journeys, as the disruption has caused delays of more than one hour and about 12 miles of congestion.
One motorist reported “biblical” rain and hail on the M25 near junction seven, as he shared a video of the flooding.
Network Rail has warned of disruption across the entire Southern and Thameslink networks until 22:00 BST – due to severe weather conditions.
It said reports of a landslip in the Merstham area in Surrey had closed the railway line via Redhill between East Croydon and Gatwick. Flooding has also shut the railway between Tattenham Corner in Surrey and Coulsdon Town in Croydon.
Meanwhile, heavy rain flooding the railway earlier on Thursday at Feniton, east Devon, is causing disruption on South Western Railway between Yeovil Junction and Exeter St Davids. Network Rail said disruption is expected until 18:00 BST.
The thunderstorms are expected to continue on Thursday evening in parts of southern England and Wales, with a drier end to the day expected in the north of England.
Skies across the UK were lit up by lightning as thunderstorms hit on Wednesday evening, following a week of hot weather.
Lightning struck a house in Wrexham, blowing out power sockets and setting fire to a curtain.
Fire crews were also called to deal with flooding incidents around Wrexham, as well as other parts of Wales including Denbighshire and Powys.
Several other places have recorded heavy downpours over the past 24 hours, such as Gnosall, West Midlands, which recorded 103.8mm of rain – over a month’s worth – in one night, according to BBC Weather.
The thunderstorms have been triggered by an extended period of hot weather in the UK, according to the Met Office.
On Wednesday, temperatures surpassed 34C in central London for the sixth day in a row – the first time that has happened since at least 1961. St James’s Park in the city saw a high of 34.6C.
Met Office forecaster Greg Dewhurst said the high temperatures could also cause flooding.
“If rain is falling on places that have been quite hot and dry, and the ground is quite hard, the rain doesn’t have anywhere to go, and from that we can see flash flooding,” he said.
On Wednesday, three people died after a passenger train derailed near Stonehaven in Aberdeenshire. It is thought the train hit a landslide after heavy rain and thunderstorms.
Scotland’s Transport Secretary Michael Matheson said extreme weather “had an impact” on the accident.
A major incident was also declared in Fife. The Scottish Fire and Rescue Service said it received more than 1,000 emergency calls on Tuesday night due to the severe weather.
The Environment Agency said 10 properties in Lancashire were also affected by flooding following storms.
It has issued flood alerts for certain areas in England and Scotland, which are separate from the weather warnings issued by the Met Office.
Meanwhile, Devon and Cornwall Police warned the south west of England is “full to capacity”, leading to “unprecedented demand” for 999 services.
The force said it saw an increase in anti-social behaviour and public order offences on Saturday and Sunday.
Assistant Chief Constable Jim Colwell said the weekend’s events, spurred on by the hot weather, had forced officers to attend a “plethora of different incidents”.
And in Sussex, more homeowners had water supplies cut off or restricted on Wednesday. At least 300 householders had already been without tap water since Friday.
Steve Andrews, head of central operations for South East Water, said more than 150 million litres of extra water were being pumped into the network as the UK heatwave continues.