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As much as a month's average rainfall could soak most of the country over two days as forecasters warned of possible flooding.
MeteoGroup said rainfall totals in England and Wales are expected to be between 30 and 50mm from Thursday to Friday, with the risk of intense thundery downpours.
A spokesman said that with average August rainfall in central and eastern England totalling approximately 50mm, parts of the country could see a month's worth fall inside 48 hours.
Senior forecaster Julian Mayes added: "A thundery area of low pressure is developing over northern Spain and this will spread warm, humid air northwards over the next two days.
"Rain is expected to reach south west England around the middle of Wednesday night. This will spread north and north east to reach most of England and Wales by midday Thursday.
"The rain may be most persistent and slow-moving in the south western half of England together with east Wales, but the risk of thunderstorms may be greatest in the eastern half of England with torrential downpours possible at times."
The Met Office said warm air moving in from the south on Thursday is 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."
Met Office severe weather warnings have been issued for Wales and all English regions except the North East on Thursday, and Wales and all English regions on Friday.
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."
Cars could be swept away
The AA warns motorists only to drive through water if they know it is 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 in 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."