Passengers are being warned not to travel on the trains as services are suspended ahead of soaring temperatures which could hit 40C.
It comes after the mercury peaked at 38.1C in Santon Downham, Suffolk, yesterday, making it the hottest day of the year and the third hottest day on record, after 38.7C in Cambridge in 2019.
The mercury will sizzle at possible highs of 41C in isolated areas today, making the country hotter than Jamaica, the Maldives and Barbados - with rail users warned of delays, cancellations and changes to train services.
Most routes across England and Wales will be affected by the hot weather today, according to National Rail, with customers told only to travel if "absolutely necessary".
The East Coast Mainline, which runs through Peterborough and Stevenage to London, will be shut this afternoon.
The Midlands Mainline between Luton, Bedford and St Pancras is also closed.
This means many train companies will not run any services and others will be very limited.
There will be no Thameslink or Great Northern trains running in any location north of London, from London Blackfriars via St Pancras, or from London King's Cross or London Moorgate on Tuesday.
Gary Walsh, route director for Network Rail in the East Midlands, said: “We’re sorry to have to close the Midland Main Line south of Bedford during the hottest part of the day.
"We want to offer the best possible service to passengers and closures are always a last resort.
"By putting in this ‘service break’ during the hottest temperatures we are protecting our ability to run a full service on Wednesday."
LNER will run no trains from south of York and south of Leeds to London King's Cross.
Greater Anglia, Southern, South Eastern, South Western Railway and Great Western Railway are among the dozens of train companies running significantly reduced services across the country.
Transport for London (TfL) said London's rail network would also be running a reduced service on Tuesday due to safety restrictions put into place to deal with the heat.
Elsewhere, council gritters were on stand-by to spread light dustings of sand on melting roads.
The RAC anticipated that the number of vehicle breakdowns on Monday and Tuesday could be up to a fifth higher than normal.
The UK Health Security Agency has issued a level 4 heat-health alert - described as an "emergency" - and the UK is under its first red extreme heat warning for a large part of England, issued by the Met Office.
Britons have been urged to stay inside during the hottest period of the day, between 11am and 4pm, and wear sun cream, a hat, stay in the shade and keep hydrated with water - and there are warnings about swimming in lakes, rivers and reservoirs.
There are also warnings of wildfires, with people asked not to use barbecues or leave litter that could spark fires in the countryside - while zoos and wildlife parks were closed to protect animals, staff and visitors.
Water companies have been experiencing "unprecedented peak demand", with people encouraged to "carefully consider" their water usage and urged not to waste it.
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