Morning train commuters face disruption after the extreme heat caused chaos on the rail network.
The train companies affected are Great Northern, Thameslink and East Midland Trains.
People are advised to avoid travelling unless the journey is essential.
Thameslink will have a reduced ability to run trains, with half of its lines unavailable.
East Midlands trains between Sheffield, Nottingham, Derby and London St Pancras International are also affected.
“East Midlands trains will be operating … on an emergency timetable due to the reduced number of lines available for trains to operate on,” the company said.
“East Midlands trains strongly urge you not to travel on the London St Pancras International / Nottingham / Sheffield route today.”
Services between Peterborough and Hitchin will not run until 7am.
In a statement, Network Rail said it had been “unable to completely fix the damage to the overhead wires between London St Pancras International and St Albans, and will continue to work on this on Friday night”.
It added: “Please check and replan your journey, as service information will be updated throughout the day.”
The weather is also causing difficulties to flights. British Airways announced that “severe thunderstorms are causing significant delays and cancellations to our operation in and out of London”.
Thousands of passengers were left stranded at major stations on Thursday, the hottest July day on record, as temperatures of up to 38.1C (100.6F) created havoc.
One passenger posted a video aboard a stationary London North Eastern Railway (LNER) service showing sweat-soaked commuters, including a man with a baby, desperately fanning themselves.
Another image shows a collapsed man being treated by medical staff outside the train.
Thunderstorms in the south-east of England were continuing to bring heavy rain, gusty winds and lightning on Friday morning.
“The thunderstorms could continue on and off through the course of the day,” Met Office meteorologist Greg Dewhurst told PA.
“There is enough energy in the atmosphere to keep it going.”
Temperatures will range from the mid-20s to the low 30s on Friday, Mr Dewhurst added.
Along Scotland’s Moray Firth coastline the temperatures will range from 25-27C, while in the south-east and East Anglia, the mercury will rise to 30-31C.
The dangers of cooling off in lakes, rivers and the sea were highlighted by emergency services after the bodies of three people were pulled from water after they reportedly got into difficulty swimming during the week’s heatwave.
The Met Office said a new record temperature for the month was set at 38.1C in Cambridge on Thursday afternoon, beating the previous record of 36.7C set in Heathrow in 2015.
But the country will see more familiar conditions return into the weekend, with the weather becoming less settled and an increased chance of rain for many eastern and north-eastern areas.
The Government’s advisory Committee on Climate Change has warned the UK is not prepared for the increase in heatwaves that is expected with global warming.