The warning remains in place for Sunday, but expands to include south-west England, all of Wales and parts of northern England. However, the heatwave continues as some parts of the UK are expected to reach highs of 27C.
According to the Met Office website there is a risk of transport disruption, flooding and power outages.
They warn: “Spray and sudden flooding could lead to difficult driving conditions and some road closures.
“There is a small chance that homes and businesses could be flooded quickly, with damage to some buildings.
“Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services.
Flood warnings are in place for southern parts of Britain
Thunderstorms should be expected in southern England
“There is a slight chance that power cuts could occur and other services to some homes and businesses could be lost.”
Most of Britain will remain warm over the weekend with southern areas more cloudy and sunshine for northern parts.
Met Office meteorologist Craig Snell forecast: “The best of the weather up towards parts of western Scotland and Northern Ireland where we see plenty of sunshine.
“But despite the drop in temperatures it will still be quite warm and humid out there, not only by day but also by night.
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Thunder and lightning are forecast for parts of southern England
“Saturday morning is going to start off on a fairly grey and murky note for many of us, thunderstorms also developing once again across England and Wales.
“Northern Ireland, western Scotland and parts of Cumbria, Lancashire, maybe parts of north Wales, throughout the day will see the best of the sunshine.
“In the sunshine temperatures will reach the low to mid-20s and even in the south, despite the cloud, because of the humidity if we see any brightness we may well see temperatures reach 25 or 26 degrees.”
Cooler temperatures should be expected in eastern parts of Britain thanks to cloud cover and a cool onshore breeze.
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Much of southern Britain is expected to be cloudy
Heavy showers are expected to hit southern parts of England
During Saturday thunderstorms are expected to head north across Britain though they’re unlikely to reach Scotland.
Mr Snell predicted: “As we head into the Saturday evening hours we may see some thunderstorms work their way a little bit further northwards but I think most of Scotland generally remaining dry.
“A little bit of a quieter interlude for the south but quite a lot of cloud around and with that humidity temperatures once again only falling down to about 17 or 18 degrees, a little bit fresher up towards the far north-west of Scotland.”
Sunday will also be grey for much of Britain with renewed risks of thunderstorms across much of the country.
Again, northern parts of the UK are expected to see most of the sunshine.
Mr Snell said: “So a grey start on Sunday, we may just see the cloud breakup at times across the south but this in turn may then allow further cloud to bubble up and risk further torrential downpours as the day goes on.
“A little bit more cloud around for Northern Ireland compared to Saturday but still some sunshine here.
Thunderstorms are expected to continue on Sunday and Monday
“The lions share on the sunshine on Sunday will be across northern and western parts of Scotland.
“So in the sunshine temperatures will reach the low to mid-20s and also down towards the south-east we could potentially see highs of 26 or 27 degrees in any sunshine.”
Further thunderstorms are expected across southern and central parts of Britain on Monday.