Flash flooding and heavy rainfall has brought an end to the UK's 33C heatwave.
Roads have already flooded in Sheffield and Strabane, in Northern Ireland, with thunderstorms and further downpours expected overnight.
Hail stones the size of £2 coins have hammered northern England, even setting off car alarms in Leeds.
It comes after the Met Office issued a 'danger to life' warning for almost the entire country.
The thunderstorm alert came into force at noon today and will last until 9am tomorrow, with warnings of 'danger to life' flooding.
A Met Office weather warning has been issued for nearly all of Scotland, western Northern Ireland, eastern Wales and all of England - apart from the south and south-west.
Met Office meteorologist Craig Snell said photographs he had seen show the hail to be about 3cm to 4cm in size.
He told the PA news agency: "Usually in the winter when we have hail it is quite small, but this time of the year, because the heat gives the thunderstorms more energy and helps keep the hail stones up in the clouds for longer, they get to grow more and then fall from the sky."
A Met Office spokesman said: "Heavy showers and thunderstorms are likely to break out on Friday afternoon and evening.
"Many places will miss the storms, but where they do occur, torrential downpours could bring 30-50 mm rain in a hour, with hail and lightning.
"The storms will slowly clear northeastwards on Friday afternoon and into the evening, though will probably not clear northeast Scotland until Saturday morning."
A Met Office rain warning has also been issued for tomorrow, with flooding of homes and roads expected.
The alert will be in force from 9am to 10pm across all of Scotland, with up to 40mm of rain expected to fall within a few hours.
The warning states: "Although intense thunderstorms become less likely for most areas across Scotland and Northern Ireland, heavy showers and more prolonged rain will continue at times, still with the possibility of thunder.
"These may bring 10-20 mm per hour for some places during Saturday, with perhaps 30-40 mm over a few hours. The ground is getting wetter now, so this may cause a few issues."
Photographs posted on social media show people in Leeds and Sheffield holding handfuls of large jagged hail stones after they fell today.
The change in weather comes as the UK experienced its hottest day of the year so far on Thursday, with the mercury rising to 33.4C (92.1F) at Heathrow Airport in west London.
