A yellow warning for thunderstorms has been issued by the Met Office for large parts of England and Wales.
The warning, which covers the majority of southern England, parts of the Midlands and most of south Wales, comes into effect from 09:00 BST on Saturday and lasts until 18:00.
Around 10-15mm of rain could fall in less than an hour while some places could see 30-40mm of rain over several hours from successive showers and storms, the Met Office says.
The UK’s weather agency also warned that frequent lightning, hail and strong, gusty winds would be additional hazards.
After a record-breaking spring, the weather has been distinctly unsettled since the start of June with wetter, windier and cooler temperatures than what is typical for the start of meteorological summer.
England experienced just 32.8mm of rain last month in what was its driest spring in more than 100 years.
Now, it appears, there could be more rainfall on Saturday than there was in the whole of May.
The weather agency warned that there was a good chance heavy showers may lead to disruption to transport, with driving conditions affected by spray, and standing water, leading to longer journey times. Train services could also be delayed.
It also said that some short-term loss of power and other services was likely, and potential lightning strikes could cause damage to buildings.
While the storms are expected to occur for much of the day, they will ease off in the west from mid-afternoon.
A yellow weather warning is the lowest level of warning issued by the Met Office. Warnings are issued on the probability of severe weather occurring as well as the impact it could have.
There will be showers in other parts of the UK on Saturday, but not as heavy as in the south.
