Torrential downpours and thunderstorms are expected to hit the country tomorrow, starting early in the morning, forecasters have warned.
As much as a month's average rainfall could soak most of the country over Thursday and Friday as forecasters warned of possible flooding and constant rain for between 24 and 36 hours.
MeteoGroup said rainfall totals in England and Wales were expected to be between 30 and 50mm from Thursday to Friday, with the risk of intense thundery downpours.
Senior forecaster Nick Prebble said: "Starting tonight, we are going to see possibly 24-36 hours of rain so parts of the country could see a month's worth of rainfall.
"Rain is expected to reach southern areas on Wednesday night and will develop across the rest of the UK throughout Thursday, with the exception of western Scotland and Ireland."
Mr Prebble added: "As it moves north there may be the possibility of flooding in some areas, but at the moment it is hard to tell where the highest risk is."
The Met Office has issued severe weather warnings for Wales and all English regions except the North East on Thursday, and Wales and all English regions on Friday.
The Met Office said warm air moving in from the south on Thursday was expected to make it feel very humid across central and southern areas of the UK.
A spokesman said: "Warmer air can carry more moisture, and as such some very heavy rain is possible.
"Current indications are that central and southern England and Wales are most likely to be affected.
"We may also have hail and thunder - this is most likely early on Thursday and then again Friday afternoon.
"As we have already seen this summer, this type of weather situation has the potential to cause sudden, localised surface water flooding and hazardous travelling conditions."
The weather system is expected to clear away to the east during Saturday, with drier, fresher weather following everywhere, with some sunshine.
The AA warned drivers to be careful.
Paul Watters, head of roads policy, said: "Flash-flooding in the summer can be hit-and-miss but potentially dangerous for anyone caught up in it.
"The AA advises drivers to keep an ear out for local weather warnings and traffic news, and be prepared to encounter flooding.
"Flooding close to where you live may present less of a problem as most drivers know which are the hazardous stretches of road and can avoid them.
"Those on holiday face a more precarious challenge and asking local people where they might run into flooding is a wise move."
The AA warned motorists only to drive through water if they knew it was not too deep - generally this would mean no more than halfway up the wheels.
Also they should not try driving through fast-moving water, such as a ford - the car could easily be swept away.
A spokesman said: "If you are driving and become stuck in flood water, it's generally best to stay in the car and telephone for help rather than try to get out - unless the water is shallow, stationary and you can see the ground beneath the water.
"If you return to find your car standing in flood water it's best to leave it and telephone for help or wait for the flood water to subside, rather than try to get to it and move it - unless the water is shallow, stationary and you can see the ground beneath the water."