Train commuters are facing major disruption this morning, after yesterday's extreme heat caused chaos on the rail network.
Affected train operators are Great Northern, Thameslink and East Midland trains.
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.
People are advised to avoid travelling unless the journey is essential.
Thursday saw the UK's second-hottest day ever recorded, with highs of 38.1C recorded in Cambridge.
The current record is 38.5C, set in Kent in August 2003.
But yesterday was the hottest-ever July day, beating the previous record of 37.7C in Heathrow in 2015.
Train company Govia Thameslink Railway has sparked a row after refusing to pay compensation to season ticket holders who took its advice not to travel during yesterday's scorching temperatures.
GTR, which operates trains in south-east England, said passengers with an annual pass are only entitled to a payout if they attempted a journey on Thursday and were delayed.
"A season ticket holder who decided not to travel would not be entitled to any refund or compensation," a spokesman for the company said.
"They would have had to make a journey and been delayed."
Thousands of passengers were left stranded at major stations on Thursday.
One passenger posted a video aboard a stationary London North Eastern Railway 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.