The Met Office has issued its first weather warning for a thunderstorm in its 164-year history, after days of sweltering temperatures.
The weather service introduced the new advisory for thunderstorms last month.
Sunday's warning said storms may develop in south Wales and south-west England throughout the day.
Forecasters warned it could lead to torrential rain, hail and lightning and there was a small chance of flooding for homes and businesses.
It also said the weather could lead to difficult driving conditions and road closures.
The yellow weather warning came into force at 06:00 BST and will remain in place until 22:00.
It comes as the country experiences a heatwave. Temperatures reached a high of 29.5C on Saturday, with Otterbourne in Hampshire recording the highest.
Thermometers are expected to hit 30C on Sunday, with higher levels of humidity predicted.
Introducing the new thunderstorm warning last month, the Met Office said: "Research found that many people felt there was a significant difference between the impacts of heavy rain in winter and those from thunderstorms.
"Therefore, we are introducing thunderstorm warnings to help communicate the potential impacts from this sort of weather, particularly through the summer months."