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), and the Met Office believes the UK could still break the all-time record of 38.5C on Thursday.
Commuters have faced disruption in areas where trains are running slower on tracks at risk of buckling.
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.
Paris has seen a record-breaking 40.6C, with much of western Europe facing all-time temperature highs.