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 with the highest recorded temperature being 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.
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.
