The Met Office has issued a yellow warning for London and south East England for Sunday, June 5.
Thunderstorms are expected to develop over parts of England and Wales from early Sunday morning, which the service says may lead to travel disruption and flooding.
It adds that fast flowing or deep floodwater is possible, with the potential for flooding of homes and businesses in a few places.
As well as potential delays and cancellations to train and bus services, the Met Office warns about difficult driving conditions and some road closures, caused by spray and sudden flooding.
Power cuts may also occur, with a chance other services to some homes and businesses may be lost.
Met Office chief meteorologist Dan Suri said: “In contrast to last night and this morning, more places within the warning area on Sunday are likely to see heavy rain and thunderstorms.
"Where thunderstorms do occur impacts from heavy rain and frequent lightning strikes are possible.
“A few places could see 15 – 20 mm of rain within an hour, with a small chance of 50 mm in an hour, most likely over parts of the Midlands, southeast England and East Anglia whilst the risk of thunderstorms over the northern part of the warning is smaller.
"Then, during Sunday afternoon new showers, some heavy and thundery, are likely over the southern or so third of the UK.
"North of the warning area in northern England and Scotland conditions will be much more settled.
"If you’re planning outside events for Sunday, please keep a regular eye on the forecast for your area for any updates.”
RAC traffic spokesman Rod Dennis said: “The fact the bank holidays coincide with the end of half-term in many places has the potential to put some extra pressure on the road network, so planning a journey carefully is important to beat the worst of any queues.
“The best way for drivers to avoid breaking down this week is to check over their vehicles before setting out – yet our research shows less than a fifth do this routinely.
"Making sure oil, coolant and screenwash are all at the right levels takes just minutes, as does ensuring tyres are free of damage and are inflated properly.
"A bit of TLC now could make the difference between a straightforward trip and one beset by a breakdown.”
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