Flash floods and lightning strikes caused chaos across the UK as a month's worth of rain fell in just an hour.
A man and his 5-year-old son were left fighting for their lives and his young girl was seriously injured after the trio were struck by lightning during a school sports day in Northern Ireland.
In south London, flash floods submerged cars and drivers had to be rescued from their vehicles after what the Met Office described as "exceptionally intense" rainfall.
Torrential rain in parts of Scotland also saw a house hit by lightning and flooding in many areas.
The Met Office issued two yellow warnings for rain in central, western and southern parts of the UK and said the intense weather could continue in to tomorrow.
After much of the UK enjoyed bright sunshine and highs of 27C on Tuesday, thunderstorms and flash floods battered the country today.
In Lisburn, County Antrim, lightning hit three members of the same family during a sports event.
The father and son were left in a critical condition while the 7-year-old girl suffered serious burns.
Forecasters estimated some 40mm of rain fell in just an hour in south London while the average rainfall for the whole of June for that area is only 49mm.
The Theatre Royal in Windsor was evacuated during the second half of a performance of One Man, Two Guvnors because of flooding.
Reports on social media said flood water had reached the front seats of the theatre.
Dramatic images saw cars being submerged and washed down the road in the Sutton and Wallington areas.
Bystander Paul Jolly said he watched as people were forced to flee on Wallington High Street as the road began to flood.
He said: "It started off with a little bit of rain....cars obviously tried to make it through under the bridge and then suddenly it was like the world was ending.
"Three cars got caught, people were being dragged out."
London Fire Brigade said it had received more than 100 flood-related calls, and had attended incidents thought to have been caused by lightning, including trees, hoardings and telegraph poles being set alight.
Two people were helped from their cars and another had to be rescued by fire crews after three vehicles became submerged in 2m of water on Manor Road in Wallington.
Grahame Madge, forecaster for the Met Office, said the downpours could continue to cause localised flooding, surface water flooding and travel disruption.
Cars obviously tried to make it through under the bridge and then suddenly it was like the world was ending
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He added: "What we have is a situation where we have warm, very moist air coming in from the continent and the heat added to the warm air causes it to rise and sees the development of these showers.
"The intensity of those can be very severe in localised areas - as we have seen in some places like London and Brighton there has been very heavy rainfall. And that has led to flooding in some areas - which is exactly why we issued the warnings."
Met Office forecaster Alex Burkill said the "very warm conditions across the country" are allowing showers, thunder and hail to develop.
He added: "The first warning covers eastern Northern Ireland across north-west and central Wales, through to the Midlands to south-eastern England.
"The second covers parts of northern England and into central and southern Scotland.
"Across those areas we could see some torrential downpours, hail and thunder are quite likely too."