Southeastern Railway customers have been told to stay at home rather than travel as Britain heads towards its hottest ever day.
With temperatures set to hit 39C on Thursday, commuters have been told that there will be a "significantly reduced service" due to speed restrictions announced by Network Rail.
Southeastern told customers: “We strongly advise you to avoid travelling, if you can.”
The Met Office has put most of the UK on "heatwave action alert" – just one step below "national emergency”.
The UK is bracing itself for what could be its hottest ever day after overnight lightning storms triggered fires and rail disruption.
According to the Met Office, there is a 60% chance the mercury could rise above the current all-time temperature record of 38.5C on Thursday.
Conditions could reach 39C in southern and eastern England.
Ahead of the record-breaking temperatures, large swathes of Britain experienced storms and lightning strikes overnight.
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North Wales was the wettest area, getting 15mm of rain in one hour.
Warwickshire Fire and Rescue Service said two engines were sent to a smoke-logged house in Bedworth, near Nuneaton, shortly before 2am after lightning struck an aerial.
Lightning also set a roof annex ablaze and caused a building to be evacuated at around 1.25am in Bowling Bank near Wrexham, North Wales Fire and Rescue Service said.
But by Wednesday morning, much of the country was already experiencing temperatures far above normal for this time of year.
Met Office meteorologist Emma Smith said: “Quite a lot of places are back up to 23 or 24 degrees already (at 5am).
“It's normally 13 or 14 degrees at this time of year, so that's 10 degrees above average.”
The highest overnight average temperature ever seen in the UK is 23.3C (73.94F) in July 1948.
Ms Smith said there is a possibility this will be beaten on Thursday night into Friday.
Temperatures in London are expected to reach above 38C (100.4F) on Thursday, which would pass the current record for a day in July - 36.7C (98.1F) - recorded at Heathrow Airport in 2015.
Northern Ireland and western Scotland will be the coolest areas on Thursday, with temperatures in the low 20s.