Thunderstorms may develop bringing torrential rain, hail and lightning to a few places, with potential disruption to travel and flooding the met office have said.
The yellow weather warning comes into force from Tuesday 6pm until 9pm on Wednesday.
There is a small chance that homes and businesses could be flooded quickly, with damage to some buildings from floodwater, lightning strikes, hail or strong winds
Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services
Spray and sudden flooding could lead to difficult driving conditions and some road closures
There is a slight chance that power cuts could occur and other services to some homes and businesses could be lost.
Area effected or could be effected:
Bracknell Forest, Brighton and Hove Buckinghamshire,East Sussex,Greater London, Hampshire, Isle of Wight, Kent Medway,Milton Keynes, Oxfordshire Portsmouth,Reading,Slough, Southampton, Surrey, West Berkshire, West Sussex,Windsor and Maidenhead and Wokingham