A new yellow warning has been issued by the Met Office for heavy showers and thunderstorms which could cause travel disruption and flooding.
Much of England and Wales is covered by the warning, which will be in force from midday until 9pm on Friday, and means flooding of a few homes and businesses is likely, trains could be delayed and car journeys could take longer, the forecaster said.
The thunderstorm warning covers most of Wales, the West Midlands, Yorkshire and Humber, South West England, North West England, North East England, London, South East England, East of England and the East Midlands.
Twelve areas are covered by Environment Agency (EA) flood alerts, meaning floods are possible.
The Met Office says the warning means flooding of a few homes and businesses is likely, leading to some damage to buildings or structures; there could possibly be some damage to a few buildings and structures from lightning strikes; a short-term loss of power and other services is possible; trains could be delayed; and there is a chance driving conditions will be affected by spray, standing water or hail.
Thunderstorms will bring 10-20mm of rain in a short period in a few places, with the potential for 30-40mm where there are repeated downpours, the Met Office said.
Meanwhile, trains have been disrupted at Knebworth towards London because of flood water making the railway potentially unsafe, National Rail said.
Disruption was expected for most of Friday morning.
There has been continued hot weather across the UK, which coincided with extremely dry conditions.
The Met Office has previously said the UK is on course for its hottest summer on record after five heatwaves, with temperatures of up to 38.1C recorded in London.
The EA said that after a National Drought Group meeting on Thursday morning, 71.3% of England remains in drought.
It said August so far has received only 34% of the expected rainfall.
Filey in North Yorkshire saw the most rainfall with 55mm between 8pm on Thursday and 7am on Friday, Met Office spokesman Oli Claydon said.
Heavy showers are forecast in the South West on Friday, as well as the thunderstorms in the warning area, Mr Claydon added.
He said: “It’s a rather unsettled bank holiday weekend.
“Saturday will start for many with a band of rain spreading north-east.
“That will be replaced by heavy showers with the potential for more thunderstorms, it will also be a windy day, particularly in the south and south-west of England.
“Sunday, a mix of sunshine and showers, winds will be generally lighter for most places than Saturday.
“Monday will continue to be changeable, sunny spells and scattered showers, there will be some dry weather around, and breezy conditions.”
Temperatures are likely to reach 20-22C on Saturday, and 20C on Sunday and Monday.
Festivalgoers heading to Reading and Leeds, Creamfields and other live music events have also been warned of unseasonably strong winds.
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