Heavy rain, thunder, lightning and hail will lash people on their way to work and possibly cause flooding, forecasters have warned.
An inch of rain could fall in just one hour on Thursday in south-east England, where a yellow thunderstorm warning has been issued by the Met Office until midday.
People should be wary of possible disruption on roads and public transport due to water and hail, as well as power cuts and damage to buildings, the forecaster said.
It comes after much of the UK basked in glorious sunshine and warm temperatures on Wednesday.
Jonathan Vautrey, of the Met Office, told the PA news agency: “We expect a large expanse area of thunderstorms to develop in the Channel, moving in from France, which may push into parts of south-east England.
“Areas in Kent, Sussex, Surrey and the Isle of Wight will widely see 10 to 15 millimetres of rain over the course of the morning.
“But if we do see the more intense thunderstorms then some locations could see 25 to 30 millimetres of rain in about an hour or so, which is quite a lot to fall in that time.
“This may produce some disruption, particularly due to the fact that it is rush hour during that period so there could be some localised disruption for anyone travelling.
“In addition to the rain we’ve got lightning, hail to around three centimetres in diameter and some quite squally winds.”
The heat from the past few days has built up, feeding more humidity into the atmosphere, which leads to the clouds that cause thunderstorms.
Mr Vautrey said areas surrounded by fields or where roads dip could be more prone to localised flooding.
