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.
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.
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.
NHS advice 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.
Try not to drink too much caffeine or alcohol, as they are diuretics that can increase dehydration.
Foods with high water content such as strawberries, cucumber, lettuce, celery and melon can also help you stay hydrated. Read our handy advice page on how o deal with the heat here.
Heatwave hits Europe
On the continent Belgium, Germany, and the Netherlands all recorded their highest ever temperatures on Wednesday.
The Met Office says heatwaves are on the increase as a result of climate change.
Higher temperatures are making extreme hot spells more likely and more intense, experts warn.