The Met Office has issued its first ever thunderstorm warning with torrential rain, hail and lightning possible this weekend.
The yellow 'be aware' warning is for South West England and South Wales and will be in effect from 6am-10pm on Sunday.
It comes as the UK continues to be in the grip of a heatwave that is expected to last for several more days.
The Met Office said the heavy rain could lead to flooded homes and businesses and difficult driving conditions.
Forecasters said: "Some 30mm-40mm of rain in an hour is possible.
"Where surface water flooding or lightning impacts do occur they are likely to be in only a few places rather than across the whole warning area.
"The greatest chance of impacts is in the afternoon, with the risk decreasing again on Sunday evening."
About a week ago, the Met Office said it would be bringing in two new types of weather warnings – one for thunderstorms, and the other for lightning.
And today is the first time the thunderstorm alert has been officially put out.
Meanwhile, despite the weather warning, millions of customers are being asked to conserve water supplies by not using hosepipes or water sprinklers.
The hot spell has put a strain on utility companies, which have been pumping millions of extra litres into their systems.
The first hosepipe ban began in Northern Ireland at 6pm on Friday, and now Severn Trent Water, which is based in the Midlands, has asked people to be "careful with their water".
Severn Trent, which has almost eight million customers, said: "We're set for another hot weekend, and, with demand for water really high, we're producing millions of extra litres.
"We're asking customers to be careful with their water and for now avoid using the garden sprinkler or hosepipe."
The company has issued some top tips on conserving water including:
:: Put garden sprinklers away – a sprinkler can use as much as 1,000 litres of drinking water in just one hour – which is more water than a family of four would normally use in a whole day.
:: Take a shower rather than a bath. It uses 40 litres less water and it's quicker.
:: Use a bucket and sponge rather than a hose. If you clean your car for 30 minutes you'll use 50 times less water with a bucket instead of a hose.
The UK had its hottest day of the year so far on Thursday in Porthmadog in north-west Wales – where the mercury hit 32.6C (90.6F). Highs of 30C-31C (86F-87.8F) are expected on Saturday.
Sky's weather producer Joanna Robinson said: "High pressure remains in charge this weekend keeping things largely settled.
"Most places will stay dry, sunny and very warm or hot, but there'll be subtle differences compared to recently.
"Ireland won't be quite as hot, with a few showers developing in the west from Saturday afternoon. The odd shower can't be ruled out over north-west Scotland too."
She added: "On Sunday, south-west Britain will be at risk of thunderstorms as humidity increases in the south.
"Any rain there will be welcome to gardeners and farmers, but torrential downpours could lead to some local flooding.
"The storms will be hit and miss, with parts of the south-west staying dry."
She also said most places will stay dry next week, with no significant rain forecast.
There will be plenty of strong sunshine again, with temperatures remaining well above the average.
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The heatwave could lead to lettuce shortages on supermarket shelves, growers have warned.
The warm weather has seen demand rising for the salad vegetable, but with temperatures above 30C (86F) the lettuce crop has stopped growing.