The Met Office has issued a yellow warning for thunderstorms as the heatwave that has seen near record temperatures for April looks set to end.
The East Midlands, North East and North West England and parts of Yorkshire and Humberside and Wales were forecast to see some flooding of homes and businesses.
The Met said this could lead to damaged buildings and structures, both from the volume of water and lightning between 4pm on Saturday and 3am on Sunday.
Motorists were also warned about the risks from driving in possible heavy rain.
Sky Weather Presenter Jo Edwards said: "We've seen temperatures close to 30C (86F) this week, and plenty of sunshine too, but thunderstorms breaking out across the country herald a change in the weather.
"The yellow warning tonight is for rain, as some of the torrential downpours could deposit an inch or so of rain in short order.
"This will give horrible driving conditions and the risk of localised flooding. Add to that some thunder and frequent lightning and it's going to be a lively night over parts of the North and South East.
"In reality it's the frontal system crossing the country west to east that will bring cooler, more changeable conditions from Monday onwards.
"The South East is likely to have another warm and sunny day tomorrow."
The warnings came as runners were advised that this year's London Marathon could be the hottest ever.
The current record is 22.7C (72.8F) in 1996 but the Met Office said temperatures at this year's event on Sunday could reach 23C (73.4F) at some weather stations in the capital.
Full sunshine was also predicted for central London in the morning prompting concerns about the effect of the heat on athletes.
Organisers of the event said they had emailed runners to warn them they needed to take the conditions into account in their race plans.
Hugh Brasher, event director, said in a statement: "We have reminded them that they should adjust their goal for Sunday and plan to run at a slower pace and, if they were planning to run in fancy dress, they should think carefully if that is appropriate in these conditions.
"There is plenty of water available and runners should drink according to their thirst and use spare water to douse their head and neck."
Race organisers added they would be putting more water, ice and shower stations along the 26.2-mile route so participants can cool down in the heat.
On Thursday, the UK enjoyed its hottest April day for 70 years when the mercury reached 29.1C (84.4F) in the capital.
On Friday, temperatures were a little lower but still made 26.1C (79F) in London's St James's Park – more than 10C above the April average – and it was at least 24C (75F) in London on Saturday.
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Forecasters said that the temperatures were not expected to be as hot in the coming days as they were last week but it would still be warm for the time of year.
It is expected to be mainly dry and fine on Monday with outbreaks of rain on Tuesday and showers on Wednesday, with temperatures a little above average, according to the Met Office.
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