Torrential downpours are set to batter Britain today, bringing the risk of flash flooding across the country.
After sweltering temperatures that touched on 32C (89.6F) yesterday, the Environment Agency has warned that up to 1.2in (30mm) of rain could fall in an hour in some areas.
The agency said that almost every county in England and Wales is at 'medium' risk of flooding, and has issued 55 flood alerts across the Midlands, the South East and the South West.
In a separate 'amber' warning of rain for England, eastern and central Wales, the Met Office has warned that heavy, thundery showers which developed overnight will move northwards during the day.
The stormy weather has caused travel disruption at Britain's biggest airport. London's Heathrow Airport said airlines were cancelling 20% of flights for a two-hour period today due to thunderstorms.
A spokeswoman said: "There is severe weather forecast at Heathrow with a 70% risk of thunderstorms, lightning strikes and hail storms. During thunderstorms the number of aircraft that can take off and land each hour is reduced.
"We have therefore requested that airlines reduce their flight schedule by 20% between 4pm and 6pm. Passengers due to travel on July 19 are urged to check the status of their flight with their airline before travelling to the airport."
Manchester Airport said an electrical surge caused by thunderstorms had led to some delays today.
General duty manager Gary Brown said: "It impacted on some lifts and baggage systems but most systems are now back up and running.
"We'll wait to see what happens this afternoon when we're expecting thunder."
Forecasters have warned that large amounts of energy in the atmosphere will lead to torrential downpours, frequent lightning, large hail and strong gusts of wind.
The Met Office said: "Not everywhere within the warning area will see thunderstorms, and indeed some spells of warm sunshine are expected at times, but where thunderstorms do form, some torrential downpours are likely with frequent lightning, large hail and locally strong gusts.
"Significant flooding is possible where these do occur from surface water as well as from small, fast-responding watercourses. The public should be prepared for the risk of disruption from any of these elements."
There is still, however, a 'yellow' warning of a heatwave for the UK today, advising people to be 'alert and ready'. The Met Office said there was a 60% chance of a heatwave continuing until tomorrow morning, with high levels of humidity.
John Curtin, director of incident management at the Environment Agency, said: "Intense heavy rainfall this weekend brings a surface water flood risk for England throughout the weekend. Flooding can happen very quickly and the public are urged to keep checking local weather forecasts and the Environment Agency website for information on a regular basis.
"We will continue to monitor the situation closely and support local authorities, who will respond to any reports of surface water flooding. We will also continue to monitor river levels closely and issue warnings where flooding is likely."
Gemma Plumb, a forecaster with MeteoGroup, said: "Today we have got some heavy and severe thunderstorms continuing to spread north across the UK. Most of the UK is going to see some heavy showers or long spells of rain during the day and severe thunderstorms.
"These could give hail, gusty winds and very frequent lightning strikes.
"It will not be as hot as yesterday, but it will still be very warm and humid."