The warning, which runs from 11am to 10pm, said flooding of homes and businesses is “likely”, with damage to some buildings from floodwater, lightning strikes, hail or strong winds.
Fast flowing or deep floodwater is also said to be “likely, causing danger to life”.
A Met Office spokesperson also said delays and cancellations to train and bus services are likely. The statement added: “Some communities likely to become cut off if roads flood. Power cuts likely to occur and other services to some homes and businesses could be lost.”
Met Office forecaster Dan Suri said “unsettled conditions” will follow the hot weather.
“The change in weather regime will see the heat of the last few days slip away from the south and east, this will be increasingly replaced with more unsettled conditions with heavy showers, thunderstorms and torrential downpours being key hazards over the UK until Wednesday,” Mr Suri said.
“Although not all places will be affected, where thunderstorms occur there is the potential for very large rainfall totals, but when that heavy rain is falling on extremely dry ground, the risk of flash flooding is much more pronounced.
"With no meaningful rainfall in some southern locations since June, soils in these areas have become baked by the sun turning them into hard almost impenetrable surfaces. Any rainfall in these areas won’t be able to soak away and instead it will wash off soils and other hard surfaces, creating flash flooding in some areas. This excess water can rapidly inundate some flood-prone areas. Particular areas of cautions are low-lying stretches of road and those areas adjoining sloping fields where water can quickly run off, creating fast-emerging hazards.”
The warning comes as Thames Water confirmed a hosepipe ban for its 15 million customers in London and surrounding areas will begin next week.
The ban will come into force from Wednesday 24 August, and is one of many being introduced across England as water companies try to save resources during very dry weather.
In a statement on its website, Thames Water said: “After the driest July on record, and below-average rainfall in 10 of the last 12 months, water levels in our rivers and reservoirs are much lower than usual.
“With low rainfall forecast for the coming months, we now need to take the next step in our drought plan,” they added.