Flooding and train cancellations have hit parts of England at the start of a week in which temperatures are expected to rise above 30C (86F).
Almost half a month’s worth of rain fell in just a few hours in some areas on Tuesday morning, with Kent and Sussex being the worst-hit.
Localised flooding was reported across the South East as commuters set off to work.
Rail operator Southern reported that all lines from Brighton through Haywards Heath were temporarily blocked due to the heavy rain and flooding on the tracks, and some passengers were delayed by more than an hour.
Coastal areas were the worst hit by the downpour, but some 0.3in (7mm-8mm) of rain in the capital caused minor flooding on some roads, according to Transport for London, and it was a wet morning commute for many.
The thunderstorms were powered up and brought to UK shores by a plume of warm air coming in from continental Europe, which is experiencing a heatwave that could bring record-breaking temperatures to France, Spain and Belgium.
Overnight in Hastings, East Sussex, between 0.6in (15mm) and 0.8in (20mm) of rain fell in just one hour, while 0.8in (20mm) fell in East Malling, Kent, over a three-hour period, according to the Met Office.
Meteorologist Simon Partridge said: “You could see some surface water issues on the roads.”
Despite the wet conditions it was a warm and muggy night for many, with temperatures of 20C (68F) in some parts of the South East.
Following the downpours, temperatures are expected to build as the week progresses and warm air moves across the Channel.
Public Health England warned that the heat could bring health risks to the elderly and people with underlying health conditions.
Peak temperatures across much of England are predicted to be well above 20C (68F), with the forecast for the Glastonbury Festival looking dry and sunny.
Heavy downpours in Scotland on Monday evening forced road closures and the suspension of tram services in Edinburgh.
Temperatures in Scotland and Northern Ireland for the rest of the week are expected to be slightly cooler, although the mercury is expected to rise above 20C (68F) on Saturday.
London could see temperatures in excess of 30C (86F) on Saturday.
Last year’s record for June, a month when the UK was hit by a series of wildfires, was 33C (91.4F) at Porthmadog in Gwynedd, Wales, while the overall June record is 35.6C (96.1F), set in Southampton in 1976.