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.
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.
Rail companies including Greater Anglia, East Midlands, LNER, and Hull Trains are advising people not to travel, while other companies are warning of delays and reduced services.
Southeastern Railway is the latest to suggest customers avoid all but “absolutely necessary” travel.
Meanwhile, Public Health England has maintained a level three heat health watch for eastern areas of England.
The highest recorded temperature on Wednesday was 33.7C (92.7F) at Cavendish in Suffolk.
On the continent Belgium, Germany, and the Netherlands all recorded their highest ever temperatures on Wednesday.
The Met Office has warned 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.