The Environment Agency has issued more than 20 flood alerts for north England, the Midlands, the South East and Wales as heavy rain is set to lash the country.
The Met Office has also issued a number of severe weather warnings, for the Central, Tayside and Fife, London and the South East, south-west Scotland, Lothian Borders, the South West, Strathclyde and Wales.
It warned that heavy rain of up to 30mm (1.2in) will fall over high ground in Scotland during the morning, with thunderstorms and gusts of up to 60mph along the south coast of England.
More top news
After the heatwave come the storms, with lightning flashes illuminating the night sky across much of the country last night.
An Independence Day event at a UK base has been cancelled by the US Air Force over security fears.
British firms predict the economy should sustain a "solid" pace of growth for the rest of the year thanks to lower oil prices and inflation.