A yellow weather warning for thunderstorms has been issued for much of the UK.
The Met Office alert covers most of southern and northern England and the Midlands.
It comes into force at 12am on Thursday and ends at 11.59pm.
"Heavy showers and thunderstorms are expected to develop on Thursday and may lead to some disruption," the Met Office said.
It comes after yellow heat health warnings were issued across most of England by the UK Health Security Agency (UKHSA).
The UK could see its hottest day of the year so far as high temperatures bring a possible official heatwave over the next few days, according to forecasters.
The UKHSA warned the conditions may have "significant impacts" on the health and social care sector, particularly across the South East and London.
All areas of England, except the North East and North West, have been included in the warning, which is in place until Wednesday.
London could see highs of 32C by Tuesday, while other parts of the country will see temperatures four or five degrees warmer than average for the time of year, the Met Office said.
Heatwave thresholds could be met across some areas from Tuesday.
The threshold is met when a location records at least three consecutive days with maximum temperatures exceeding a designated value, according to the Met Office.
This is 25C for most of the UK, but rises to 28C in London and its surrounding area, where temperatures are typically higher.
Met Office spokesperson Nicola Maxey said London and the South East could see temperatures around 30C today, while it will be cooler nearer the coast - before conditions become increasingly unsettled during the middle of the week.
She added: "As we move into the middle of the week we start to see an increasing risk of thunderstorms developing, particularly in parts of England and Wales, and the rain is such that it could cause some local disruption.
"This is the sort of rainfall that could cause surface water flooding and heavy rain that takes a while to clear away.
"There could also be some lightning potential for hail and gusty conditions."
Last week, the Met Office revealed how the average number of days of 30C heat or above had trebled in Britain.
The UK's climate is still getting sunnier, wetter and warmer, it said in an annual review.
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