A yellow weather warning for thunderstorms is in force across England and Wales - with a risk of flash flooding and travel disruption.
A band of heavy showers and thunderstorms is expected to move northwards throughout Wednesday morning, the Met Office said.
It said warm and humid conditions will bring an increasing chance of heavy showers, thunderstorms and longer spells of rain.
The yellow warning for thunderstorms began at 3am and will last until 10pm on Wednesday.
It covers much of England, including London and the South East, the Midlands, East Anglia, as well as parts of the North West, South West, and eastern parts of Wales.
Met Office chief forecaster Steve Willington said: "As we move into Wednesday there is a growing risk of heavy rain and thunderstorms, with warm and humid air helping to trigger heavy showers and thunderstorms.
"Some places could see significant amounts of rain falling in a short period of time, bringing the potential for localised flooding, travel disruption and frequent lightning.
"On Thursday, slow-moving heavy and thundery rain may bring prolonged rainfall to some areas, increasing the risk of localised impacts.
"There remains some uncertainty around where the heaviest rain will fall, so it is important to keep up to date with the latest forecast and warnings."
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The Met Office warned showers and thunderstorms on Wednesday afternoon "may be intense", with 20-30mm of rain falling in less than an hour, and 40-50mm possible in two to three hours in some spots, leading to some surface area flooding.
Spray and sudden flooding could lead to difficult driving conditions and some road closures, it added.
Large hail and frequent lightning are also possible before the showers and thunderstorms drift northwards and ease during Wednesday evening.
Another yellow weather warning for thunderstorms has been issued from 6am to 10pm on Thursday, covering large parts of England including the Midlands, East Anglia, the North West, Yorkshire, the South West, as well as much of Wales.
There is a "small chance of fast-flowing or deep floodwater causing danger to life", the weather service said.
It added there was also a small chance homes and businesses could be flooded quickly, with damage to some buildings from floodwater, lightning strikes, hail or strong winds.
The Met Office said 20-30mm of rain in three hours is "expected fairly widely", while up to 50mm in a 12-hour period can be expected in places.
There is also a small chance some places may see more than 50mm of rain in three hours and more than 75mm through 12 hours "especially over east facing high ground".
Lightning and hail of 1-2cm will also be hazards on Thursday, the Met Office added.
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