Severe weather warnings have been issued for much of the country, with heavy showers expected to continue over the coming days.
Thundery downpours have soaked parts of the UK, and forecasters have warned of the risk of flooding in Wales, the North West, Yorkshire and Humber, the West Midlands, the East Midlands, and the South West and South East.
Many of these areas could see the showers lasting into Thursday, with some turning into thunderstorms.
The warnings have been issued as lightning strikes were recorded in Wales, north London and south-east England.
The Environment Agency has issued a dozen flood alerts and West Midlands Fire Service also issued warnings about the conditions as it responded to multiple flash floods.
The Met Office said a slow-moving area of low pressure, mixed with rising temperatures, was causing the "unstable" weather.
Forecaster Alex Burkill said: "Already we've had thunder and lightning over parts of Wales, north London and parts of Essex. I would be very surprised if we did not have more thundery activity over the bulk of the rest of the country.
"What we have is the low pressure tracking eastwards across the UK making things unsettled and unstable. The warm temperatures are causing the air to rise, allowing large clouds to form and so the warmth is linked to the fact that it is unsettled."
The Met Office yellow weather warning said: "Heavy showers will break out today, leading to some prolonged downpours. Some locations will miss these but where they do occur, please be aware of possible localised flooding and disruption to travel. Lightning could be an additional hazard."
On flash flooding, Burkill added: "It is similar to what we had last week in Birmingham and parts of London. It is not a huge amount of rainfall, but how quickly that rain falls.
"Looking at the UK average for June, it's not exceptionally wet."