Britain's hottest week in years is threatening to become a wash-out, as parts of the South West were put on flood alert.
The Environment Agency has warned the area and South Wales were set to be hit by flash floods during the day.
Tony Conran, forecaster for MeteoGroup UK, the weather arm of the Press Association, said: "There remains a possibility for the hottest day of the year but for other parts there could be floods.
"There's an area of very heavy rain moving in towards the hills of Bodmin and Exmoor. Anywhere in Cornwall, Devon and Somerset there could be a hell of a lot of rain."
An Environment Agency spokesman added: "The Environment Agency and Met Office are warning homeowners, businesses and drivers in south west England and South Wales that heavy, thundery rain could lead to flash flooding, particularly from surface water drains and small watercourses."
Forecasters had previously raised the heatwave warning alert level from two to three.
The current weather spell is the hottest since July 2006, with yesterday the hottest of the year so far at 31.8C (89.2F).
The heatwave plan alert is in four stages, with green level one signalling "summer preparedness and long-term planning".
Level two is amber and signals "alert and readiness", while three is red for "heatwave action".
Level four is classed as "red emergency".Reuse content