Train passengers are being urged to only travel if “absolutely necessary” on Thursday and Friday in areas most affected by extreme heat.
Network Rail has confirmed that trains will operate at reduced speeds and services will be cut for "safety reasons".
Heat-related issues that can affect the railway include buckled rails, sagging overhead electric wires, and lineside fires. Steel rails can also exceed 50C when the air temperature is 30C.
Network Rail has urged passengers to "only travel if absolutely necessary" if at least part of their journey is within the Met Office’s red warning zone on Thursday, or the amber warning zone on Friday.
Thursday’s red zone covers large parts of southern England and southern Wales, while Friday’s alert stretches from the South East up to the North West and South Yorkshire. Anyone travelling elsewhere on Friday "should still check before they travel", Network Rail added.
Provisional figures suggest the temperature on Wednesday reached 36.1C in Gosport, Hampshire.
Jake Kelly, deputy chief executive of Network Rail, said: “We know how important it is for people to get where they need to be, but this spell of very hot weather will make journeys more challenging across the network.
“Very high temperatures, like we are seeing in Great Britain and across parts of Europe, put significant strain on the railway, so our priority is to keep passengers safe while keeping services running.
“We’re asking passengers to think carefully about their plans and only travel if it’s absolutely necessary in the areas most affected areas by the extremely high temperatures that are being recorded.”
Passengers who do need to travel are being urged to plan ahead and carry a bottle of water.
Chiltern Railway is running fewer than half its usual services between Wednesday and Friday, affecting London Marylebone services to and from Birmingham, Oxford and Aylesbury via Amersham.
The operator acknowledged “there may be significant delays and very busy conditions”.
Avanti West Coast has cut one train per hour on its Birmingham, Liverpool and Manchester routes between Tuesday and Friday.
LNER, which runs services on the East Coast Main Line, told passengers “we strongly recommend that you do not travel” on Thursday or Friday, and those planning to take a train on Saturday should check for service updates.
Many operators have eased restrictions on tickets, meaning they can be refunded without a fee or used on alternative dates.
