There is also a risk of flooding from rising groundwater across parts of Wiltshire and Somerset, including Salisbury Plain
More flood misery is predicted for parts of Britain after nearly a month's worth of rain fell in just three hours.
Thunderstorms and persistent showers drenched the south coast of England and the Environment Agency (EA) warned areas of Dorset and Kent could be at particular risk.
There is also a risk of flooding from rising groundwater across parts of Wiltshire and Somerset, including Salisbury Plain, the weather experts warned.
In Somerset, early heavy rain has caused flash flooding in Shepton Mallet, near Glastonbury, when up to 30mm of rain fell in two hours. The EA has deployed pumps to the area.
There are currently five EA flood warnings in place, in the South West and the Midlands, and 23 flood alerts across the country.
The Met Office has issued amber warnings for severe weather across swathes of Britain. Craig Woolhouse, head of flood incident management at the EA, said: "The combination of saturated land, high river levels and further forecasts of deluges of rain mean people must remain vigilant."
Paul Knightley, forecaster at MeteoGroup, the Press Association's weather division, said the area around Christchurch, near the Dorset coast, saw about 52mm of rain in just three hours. He said the rain would start to ease off until Thursday evening, but further showers could bring up to 20mm of rain, creating more risk of flooding.
The forecaster said: "The showers and thunderstorms that are there now, they will start to clear away. It is mostly dry throughout the night but there will be more rain tomorrow evening and quite heavy rain tomorrow night."
Meanwhile, the Olympic flame route in Dorset on Thursday, Friday and Saturday could be affected by the threat of flooding, the county council said.
Angus Campbell, chairman of Dorset's Olympic Board, said: "All Dorset agencies are working very closely with the Met Office and the Environment Agency so that we can make decisions as quickly as possible on the route the Olympic flame will take. Our intention, where possible, is to use the published route but we have alternatives in place if the weather causes further difficulties."