Brits have been warned to batten down the hatches as an amber alert for thunder storms has been issued, with flooding likely across several counties.
Eleven areas in the east, south east and London have been warned by the Met Office to expect heavy rain and storms between 11am and 10pm today, with as many as two inches forecast to fall.
After weeks of dry conditions the deluge is expected to cause flooding in several areas, with power cuts also feared.
The Met Office warned: "Heavy showers and thunderstorms are expected to break out today leading to some flooding and disruption.
"Some places will miss these, but where they do occur, 30 to 50 mm of rain could fall in less than an hour and a few places may see in excess of 100 mm in a few hours where storms are slow moving. Lightning and hail will be additional hazards.
"Showers and storms will slowly die out this evening."
Among the areas affected by the amber alert is London, with residents and commuters warned of possible disruption owing to the conditions.
Brits are warned it may cause delays or cancellations to bus and train services caused by deep and fast flowing flood water.
People in rural areas may also be cut off by the deluge of water after weeks of bright sunshine and high temperatures.
A separate yellow thunderstorm warning remains in place for nearly half the country, with forecasters extending it further north to include southeast Wales.
A spokesman added: "The yellow area has been extended northwestwards to cover more of England and moving into southeast Wales, the start time brought forward, and the likelihood of impacts increased."
Although rain is likely to sweep across the UK on Wednesday, there is set to be sunny weather in the north while temperatures are predicted to peak in the south east with 25C highs.
It is a similar outlook for Thursday where it will be slightly warmer and could hit 26C.
Met Office forecaster Clare Nasir earliersaid: “A fine start to the day for Scotland, a light breeze and some sunshine, that sunshine extends down to northern counties of England.
"The cloud will break up across Northern Ireland, a good day here to be out and about with some brighter weather and you can see some sunshine just appearing across the north west of Wales.
"But here’s that thundery rain across the bulk of Wales and extending to Lincolnshire and some clusters of thunderstorms developing and moving very slowly across the home counties, central and southern England as well as East Anglia.
"So through the day this is where the lively weather will be.”
Areas affected by the amber alert
East of England
Essex
Southend-on-Sea
Suffolk
Thurrock
London & South East England
Brighton and Hove
East Sussex
Greater London
Kent
Medway
Surrey
West Sussex
Areas affected by the yellow alert
East Midlands
Leicester
Leicestershire
Lincolnshire
Northamptonshire
Rutland
East of England
Bedford
Cambridgeshire
Central Bedfordshire
Essex
Hertfordshire
Luton
Norfolk
Peterborough
Southend-on-Sea
Suffolk
Thurrock
London & South East England
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
Wokingham
South West England
Bath and North East Somerset
Bournemouth Christchurch and Poole
Bristol
Cornwall
Devon
Dorset
Gloucestershire
Isles of Scilly
North Somerset
Plymouth
Somerset
South Gloucestershire
Swindon
Torbay
Wiltshire
Wales
Blaenau Gwent
Bridgend
Caerphilly
Cardiff
Merthyr Tydfil
Monmouthshire
Neath Port Talbot
Newport
Rhondda Cynon Taf
Torfaen
Vale of Glamorgan
West Midlands
Herefordshire
Warwickshire
West Midlands Conurbation
Worcestershire