Britain is expected to set a new record for its hottest ever day as temperatures climb to 39C.
According to the Met Office, there is a 60 per cent chance the mercury could rise above the current all-time high of 38.5C on Thursday.
Forecasters are warning of thunderstorms towards the end of the day, which could in turn trigger travel delays, flash flooding, and power cuts.
Network Rail has also warned commuters that speed restrictions may be introduced in areas where tracks are at risk of buckling as the metal heats up to 50C in the blazing sunshine.
Please allow a moment for the live blog to load
Disruption to Network Rail services is likely to continue until tomorrow morning, a spokesman has said.
Nick King, network services director at Network Rail said: "There were a number of heat related incidents across the railway on Thursday, 25 July.
"Our teams have been working flat out to fix the issues, however we are sorry that disruption is likely to continue into the morning (Friday, 26 July).
"We are advising passengers to check with their train operators or the National Rail Enquiries website before travelling."
Temperatures are starting to fall, the Met Office has said, so it is unlikely we will see temperatures above 38.1C today.
London's Central Line has been recorded as being hotter than body temperature today:
London's Central Line is hotter than body temperature
The mercury hit 36.6C between Mile End and Stratford, finds Simon CalderHeavy thunderstorms are moving towards Kent, the Met Office has warned.
Nick King, network services director at Network Rail, 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."
Overhead electric wires between London Euston and Watford Junction have been damaged by the heat, disrupting Virgin Trains services.
A spokeswoman for the operator said: "Due to extensive disruption on the network today, any Virgin Trains customers who would prefer to postpone their travel can use their tickets on Virgin Trains services tomorrow.
"A full refund will be available to those who choose not to travel."
The scorching temperatures have caused damage to overhead electric wires between London St Pancras and Luton, blocking all lines.
This is affecting East Midlands Trains and Thameslink services.
East Midlands Trains posted a message on Twitter urging passengers "DO NOT TRAVEL" and warned it had been unable to secure ticket acceptance via alternative routes.
Thameslink said "you are strongly advised not to travel", and said journey times will be extended by up to 90 minutes.
38.1C has been recorded at Cambridge, according to the Met Office.
The temperature to beat is 38.5.
Eurostar said delays of between 30 minutes to an hour are also expected on the Brussels route today because of speed restrictions.
A company spokesperson said: “Delays between 30 minutes to an hour are expected on the Brussels route today, as a result of speed restrictions set by the infrastructure provider due to high temperatures.”
Passengers are being offered the opportunity to cancel.
There is pandemonium at London St Pancras as all lines are blocked because of damage to overhead electric wires between Luton and St Pancras International.
Network Rail said: “Extreme heat can cause overhead wires to sag and become damaged by fast moving trains.”
All lines are blocked between London St Pancras international and Bedford.
Bedford to Brighton services will not run until further notice
St Albans to Sutton services will not run until further notice
Services from Bedford will only run as far as St Albans
Friends of the Earth has urged new prime minister Boris Johnson to put responding to the climate emergency at the heart of his new government after the UK’s maximum temperature record for July was broken today.
Chief executive Craig Bennett said: “The dangerous heatwave battering Britain is a stark warning to Boris Johnson on the urgent need to end the country’s reliance on climate-wrecking fossil fuels.
“Developing the UK’s vast renewable power potential, insulating our heat-leaking homes and investing in modern, carbon-free transport systems would slash emissions, create thousands of new jobs and put the UK at the forefront of building a cleaner, safer future.
“The first hundred days are crucial – will the new Prime Minister take decisive action to deal with the climate emergency, or dish up more hot air?”
Alongside cutting emissions in the first instance, Friends of the Earth is calling for doubled tree cover to protect people from the impact of extreme weather such as heatwaves and to help absorb carbon from the atmosphere.
Today is now officially the second hottest day on record after 37.9C was recorded at Heathrow Airport.
It quickly surpassed the 37.7C registered at in Kew Gardens, London, and Writtle, Essex, less than an hour earlier.
The previous second highest record of 37.1C was set on 3 August 1990.
Police had to be called to an outdoor swimming pool after hordes of overheating Londoners tried to force their way in.
About 500 people tried to storm Brockwell Lido in southeast London at about midday today when waiting times topped three hours.
There were similar scenes elsewhere in the capital - with Parliament Hill Lido forced to refuse further entry just before noon due to overcrowding.
Police were repeatedly forced to attend the pool after fights broke out in the queue, and eventually the police decided to leave an officer effectively on guard to prevent any more disruption.
Temperatures have climbed to 37.7C at Kew Gardens, in London, and Writtle, in Essex.
Not far off the all-time record of 38.5C.
Germany has broken its all-time highest temperature record for the second day running.
According to the Germany Weather Service, 40.9C was recorded in the northwestern town of Lingen today.
It surpassed the previous record of 40.5 degrees measured in western Germany just yesterday.
While many of us across the UK melt in today's heat, let's all spare a thought for the cyclists taking part in the Tour de France.
With near-record Tour temperatures of 40C in the shade and 60C on the road, the world's best cyclists are having to take drastic measures to cope.
Team Ineos packed 10 water bottles per rider in preparation for today, but it wasn't enough.
Read more from sports correspondent Lawrence Ostlere here:
Madame Tussauds in Blackpool has had to move wax figures, including those of Freddie Mercury and Ariana Grande, to stop them from melting in the heat.
Staff were forced to take “drastic action” and have been busy moving the waxworks away from windows at the attraction.
Ariana and Freddie were said to be at particular risk because their fingers and hands are outstretched with exposed digits, meaning there is a larger surface area of wax that could potentially melt.
People have been out soaking up the sun on what is expected to be the hottest day of the year.
Temperatures have already hit 35C in some places, but could rise to 39C by the end of the day.
Families have been relaxing on the beach in Pembrokeshire, in Wales, while Londoners have enjoyed the sunshine in Hyde Park and while watching the cricket at Lord's.
Families have been relaxing on the beach in Pembrokeshire, in Wales, while Londoners have enjoyed the sunshine in Hyde Park and while watching the cricket at Lord's.
