The UK is expecting its hottest day on record on Thursday, with temperatures of up to 39C (102.2F) forecast in southern and eastern England.
The Met Office said there was a 60% chance of the current record of 38.5C (101.3F) from August 2003 being broken.
Network Rail warned of disruption in areas where tracks were at risk of buckling – with many rail firms advising passengers not to travel.
Temperatures topped 30C in south-east England on Wednesday.
The highest recorded temperature was 33.7C (92.7F) at Cavendish in Suffolk.
Elsewhere on Thursday, parts of Scotland could see temperatures close to 30C, while parts of Wales could also reach 30C. A weather front close to Northern Ireland will keep it cooler.
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
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.
How will it impact travellers?
Speed restrictions are in place on some train routes because of the high track temperatures.
Great Western has cancelled some trains between London, Cardiff and Swansea because of the heat.
Network Rail, which manages the rail network infrastructure, says tracks can get up to 20 degrees hotter than the air temperature.
This may cause delays and cancellations to journeys across the whole Gatwick Express, Great Northern, Southern and Thameslink networks until the end of the day.
Southeastern Railway is the latest to suggest customers avoid all but “absolutely necessary” travel.
Even those travelling by car could find their journey less enjoyable as FM and AM radio signals can be disrupted in hot weather.
This happens when heat causes the signals from local stations to travel further and cause interference outside their usual range.
How to stay safe
Public Health England has maintained a level three heat health watch for eastern areas of England.
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.
If your body’s temperature goes over 40C (104F), heat-stroke can set in, which requires urgent medical help. Danger signs include sweat stopping – the person may feel hot, but dry – and breathing difficulties.
Physical thirst is not a very reliable indicator of how dehydrated people are (urine colour is better), so try to drink plenty before you feel parched. Read our handy advice page on how to deal with the heat here.
Heatwave hits Europe
On the continent Belgium, Germany, and the Netherlands all recorded their highest ever temperatures on Wednesday.
A red alert has been issued for Thursday in Northern France – with temperatures of 41C forecast in Paris.
Belgian, German and Dutch temperature records could be broken for the second time in two days.
On Wednesday, a Eurostar train from Belgium to London broke down, trapping passengers, while French reports suggested five deaths might have been linked to the heatwave.
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 to 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.
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