The Met Office has declared an amber weather warning for hail, thunder, lightning and floods in southern England.
Heavy showers and thunderstorms are on the way as some parts get a brief respite as the heatwave continues.
But the change in weather may lead to localised flooding and travel disruption.
The Met Office say London, Cambridgeshire, Essex and Suffolk can all expect conditions to alter.
Greater London and parts of Kent are also at risk, they say.
Heavy showers and thunderstorms are expected to persist for two to three hours in some areas.
Hail, frequent lightning and gusty winds are also possible.
They have told of what to expect, warning that “flooding of homes and businesses is likely and could happen quickly”.
The Met Office went on: “Spray and sudden flooding probably leading to difficult driving conditions and some road closures.
“Where flooding or lightning strikes occur, delays and some cancellations to train and bus services are likely.
“Power cuts likely to occur and other services to some homes and businesses could be lostFurther details
A few places may see 40 to 60 mm of rain within two hours.