Commuters are enduring chaos and disruption on the railways as the UK swelters on its hottest July day on record.
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.
In Edinburgh, the mercury rose to 31.6C, making it the hottest day on record in Scotland's capital.
It is also the second hottest UK day on record, beating the 37.1C recorded in August 1990.
Exceptionally high temperatures have gripped Scotland as well as much of central, eastern and southern England as a plume of hot air pushes north from the Continent.
New high temperature records have also been set in parts of Belgium, Germany and Holland as a heatwave grips western and central Europe, pushing the mercury above 40C.
Experts at the Met Office say the current weather pattern is driving hot air from the south, but there is "no doubt" climate change is playing a role in the intensity of the heat being witnessed.
The scorching temperatures are causing chaos on the rail network.
A reduced timetable in the South East came into force at midday as Network Rail implemented speed restrictions amid fears tracks could buckle in the heat if trains travel too fast.
Speed limits on most commuter lines have been reduced from 60mph to 30mph.
But the extreme conditions also caused damage to overhead electric wires, blocking all lines between London and Luton.
Overhead wire failures also caused disruption between London and Watford, between Preston and Carlisle and in the Birmingham area.
Many operators have urged passengers not to travel as services are delayed and cancelled.
Network Rail's network services director Nick King said: "We have a number of heat-related incidents across the rail network this evening that are causing disruption to services.
"We are sorry that some passengers are experiencing uncomfortable conditions and inconvenience.
"Our teams are working flat-out to fix the issues as quickly as possible and get people on the move.
"We're asking anyone travelling this evening to check with their train operators or visit the National Rail Enquiries website to see how their journey is affected."
Further travel disruption could be caused by thundery downpours prompted by the sweltering temperatures, with flash flooding and even power cuts possible.
A yellow warning for thunderstorms has been issued for most of England except the South West, and parts of Scotland until 4am on Friday.
Met Office chief meteorologist Steve Willington said: "We're already seeing thunderstorms being triggered by today's hot weather and we'll continue to see thunderstorms breaking out this evening and overnight across wide areas of the UK.
"Also tonight it's going to be very warm across central, eastern and south-eastern parts in particular as temperatures fall no lower than 23C to 24C in places, which could see further temperature records broken."
The Met Office said the current record for the highest overnight temperature in the UK is 23.9C, set in August 1990.
On Wednesday night some places experienced a "tropical night" with temperatures staying above 20C, and that is likely to be repeated on Thursday into Friday.
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.