A yellow weather warning has been issued across London, the South East and the South West of the country.
The Met Office has issued a yellow thunderstorm warning from 10am to 9pm on Thursday, expecting heavy showers to bring disruption.
The forecaster said thunderstorms and heavy showers are expected to develop during Thursday morning and through the afternoon.
A few areas will see “torrential downpours”, the Met Office said, with 25 to 35mm of rain falling within an hour, and potentially 60mm within two hours. Frequent lightning and hail will bring additional hazards, it warned.
“Storms will tend to become more confined to the south and east of the warning area later in the afternoon before dying out during the evening,” it said.
The warnings cover major counties and cities such as London, Suffolk, Brighton, Kent, Oxfordshire, Bristol, Portsmouth and Southampton.
People living in areas at risk of flash flooding are advised to “consider preparing a flood plan and an emergency flood kit” to minimise potential damage, the Met Office advised.
Strong winds could also pose a hazard, and residents have been told to secure objects including bins, trampolines, tents and garden furniture before gusty conditions arrive.
Motorists have been urged to check road conditions and public transport timetables before travelling, and to amend plans if necessary to avoid getting caught out by severe weather.
The Met Office has also told households to prepare for possible power cuts by gathering essentials like torches, spare batteries and mobile phone power packs.
In its UK-wide forecast, the Met Office said there will be a few showers on Friday, mainly in the East. Saturday will remain largely dry with warm sunny spells, and locally heavy rain will spread on Sunday. Temperatures will be near normal throughout the weekend, the forecaster added.
Here is a full list of areas covered by the weather warning:
East of England
- Central Bedfordshire
- Essex
- Hertfordshire
- Luton
- Southend-on-Sea
- Suffolk
- Thurrock
London and south-east England
- Bracknell Forest
- Brighton and Hove
- Buckinghamshire
- East Sussex
- Greater London
- Hampshire
- Isle of Wight
- Kent
- Medway
- Oxfordshire
- Portsmouth
- Reading
- Slough
- Southampton
- Surrey
- West Berkshire
- West Sussex
- Windsor and Maidenhead
- Wokingham
South-west England
- Bath and North East Somerset
- Bournemouth Christchurch and Poole
- Bristol
- Dorset
- Gloucestershire
- North Somerset
- Somerset
- South Gloucestershire
- Swindon
- Wiltshire