The UK has had its hottest July day on record, with temperatures reaching 38.1C (100.6F) in Cambridge.
The previous record for July was 36.7C (98F), with Thursday’s top temperature falling just short of the all-time high of 38.5C, reached in 2003.
Commuters are facing disruption after Network Rail reported a number of heat-related incidents on the railway.
Forecasters have warned the heat could lead to downpours in some areas, causing more disruption to travellers.
The Met Office said the temperature reading from Cambridge was only the second time the UK had gone over 100F.
The July heat record was first broken at around 13:30 BST on Thursday when the Met Office said the temperature at Heathrow reached 36.9C.
The previous hottest July day was recorded on 1 July 2015, also at Heathrow, with a temperature of 36.7C.
Elsewhere in Europe, Germany reached a new temperature record of 41.5C on Thursday, with other countries in the region facing the possibility of new all-time highs.
The UK’s current highest ever temperature was recorded at Faversham, Kent, in August 2003.
How is it affecting travellers?
Network Rail said heat-related incidents were causing disruption across the rail network and warned people to check with train operators or the National Rail website before travelling to see how their journey was affected.
It said it was working “flat out” to fix the issues as quickly as possible.
The Rail Delivery Group, which represents train operators and Network Rail, said the disruption was particularly bad in London and the South East and warned problems could continue until the weekend.
Robert Nisbet, director of nations and regions at the group, said 20 out of 26 train companies had been affected by cancellations and speed restrictions.
“We believe there could be roll-on impacts tomorrow, because many of the trains will be in the wrong places, many of the rail staff will be in the wrong places and it will take time to re-set, to stabilise the timetable again,” he said.
“We could see disruption up until the weekend.”
It was ‘like a sauna’
Vivienne Tsui, 23, told the BBC she was waiting to be evacuated from a train in “unbearable” heat after it stopped just after departing from Euston station at around 13:30.
Passengers were told the train, bound for Manchester Piccadilly, had suffered a power shortage and there was no air conditioning, she said.
“It’s been unbearable, like a sauna. I had to get out of my seat because I felt like I couldn’t breathe,” she said.
The University of Manchester worker said she did not know whether she would be able to get back to Manchester on Thursday and would have to contact her work.
“I don’t know what arrangements have been made for us once we get off this train,” she added.
Travellers face delays and cancellations to journeys between London Euston and Watford Junction until at least 19:00 after overhead electric wires were damaged.
East Midlands Trains and Thameslink are advising passengers not to travel after damage to overhead wires between London St Pancras and Luton.
Speed restrictions are in place on some train routes because of high track temperatures.
Great Western has cancelled some trains between London, Cardiff and Swansea because of the heat and routes between London and Scotland are also affected.
Delays and cancellations are also expected for journeys across the Gatwick Express, Great Northern, Southern and Thameslink networks until the end of the day.
Other rail companies, including Greater Anglia, East Midlands, LNER and Hull Trains are advising people not to travel.
Southeastern Railway is the latest to suggest customers avoid all but “absolutely necessary” travel.
What’s the forecast?
On Thursday, Scotland recorded its hottest day of the year at 31C, while in Wales it was forecast to exceed 30C. A weather front close to Northern Ireland will keep it cooler.
Later on Thursday, eastern Scotland and the north and east of England could see rain, with a Met Office yellow warning for thunderstorms in place from 15:00 BST on Thursday into early Friday morning.
There are warnings that the storms could trigger travel delays, flash flooding, and power cuts.
Heatwave hits Europe
On the continent, Germany set a new temperature record of 41.5C on Thursday.
Belgium and the Netherlands also faced the possibility of new all-time highs, a day after recording their highest-ever temperatures on Wednesday.
A red alert has been issued for Thursday in northern France – with temperatures of 41C forecast in Paris.
French reports suggest five deaths might have been linked to the heatwave.
How to stay safe
People are being urged to help out vulnerable friends, relatives and strangers in the heat.
Age UK has issued guidance for older people to stay safe while homeless charities are handing out water and sun cream.
Public Health England has maintained a level three heat health watch for eastern areas of England.
In Bristol, thousands of people have been left without water after a water main burst, with Bristol Water saying the problem might have been caused by the heat.
Homeless charities are handing out water and sun cream to rough sleepers across the country and Public Health England has issued a level-three heat health watch for eastern areas of England.
Police have also warned of the risks of cooling off in open water, after three bodies – believed to be swimmers – were recovered in London and Gloucestershire.
Britain is not used to such extreme temperatures, which means some people could be vulnerable to heat exhaustion.
The NHS recommends keeping all babies under six months out of direct sunlight, and older infants should be kept out of the sun as much as possible, particularly between 11:00 and 15:00.
They should be kept in the shade or under a sunshade if they’re in a buggy or pushchair. Sun cream with a high sun protection factor should be applied regularly – particularly if children are in water.
All children should be given plenty of fluids and the NHS says babies who are being breastfed may want to feed more than usual, but will not need water as well as breast milk.
NHS advice also says people should cool off immediately if they show the following symptoms: headaches, feeling dizzy, loss of appetite, nausea, excessive sweating, cramps, fast breathing and intense thirst.
For more information on how to keep safe in the heat, read:
Is the heatwave caused by climate change?
While extreme weather events like heatwaves occur naturally, “research shows that with climate change they are likely to become more common, perhaps occurring as regularly as every other year”, the Met Office says.
It conducted a study last year that found that the UK was now 30 times more likely to experience heatwaves compared with the year 1750, because of “the higher concentration of carbon dioxide (a greenhouse gas) in the atmosphere”.
Records going back to the late 19th Century show that the average temperature of the Earth’s surface has increased by about one degree since industrialisation.
A climatology institute in Potsdam, Germany, says Europe’s five hottest summers since 1500 have all been in the 21st Century.
Scientists are concerned that rapid warming linked to use of fossil fuels has serious implications for the stability of the planet’s climate.
What are the UK’s current record temperatures?
The current record temperatures across the UK are:
- England and UK: 38.5C (101.3F) in Faversham, Kent on 10 August 2003
- Scotland: 32.9C (91.2F) in Greycrook, Borders on 9 August 2003
- Wales: 35.2C (95.4F) in Harwarden Bridge, Flintshire on 2 August 1990
- Northern Ireland: 30.8C (87.4F) in Knockarevan, County Fermanagh on 20 June 1976 and Shaw’s Bridge, Belfast on 12 July 1983