All Great Northern and Thameslink trains which run north of London are cancelled tomorrow due to "extreme heat".
Govia Thameslink Railway, which runs trains throughout the Home Counties, has opted to suspend all services to the north of London's Zone 1 on Tuesday, July 19.
The decision to cancel trains follows a day of disruption on the UK's rail network as temperatures hit 36C throughout Bedfordshire, Cambridgeshire, Hertfordshire and London.
The Met Office has a red weather warning for "extreme heat" on Tuesday, with the mercury set to hit 39C in the region.
A statement released on Monday read: "There is significant disruption following a number of heat related incidents on the railway.
"On Tuesday, July 19, do not travel on routes to the north of London."
The firm previously announced that it would not run any trains out of London King's Cross and London Moorgate, although some limited trains would run on other routes.
Services have been scaled back further, with no trains between:
- Bedford and London St Pancras (via Luton Airport and St Albans)
- Peterborough and London St Pancras (via Huntingdon and Stevenage)
- King's Lynn, Cambridge and central London (through Ely, Royston and Stevenage)
- Welwyn Garden City and central London
- Stevenage and London Moorgate (via Hertford North)
This will effectively result in the closure of several stations in some of the home counties' large towns and cities, such as St Albans, Hatfield, Huntingdon and Royston.
A Thameslink spokesperson said: "On routes south of London you should only travel if your journey is absolutely essential.
"Please do not use our trains to go to the beach.
"There is a high risk of significant disruption.
"Even if you are able to travel in the morning, you may find there is significant disruption to return journeys in the evening, so if you must travel because you're making an essential journey you should plan a way home without using the train service."
They added: "You can use tickets dated for Monday, July 18 and Tuesday, July 19 to travel with Southern, Gatwick Express, Thameslink or Great Northern on Wednesday, July 20 and Thursday, July 21 instead.
"Season ticket and flexi ticket holders - if you are unable to travel or choose not to travel, you can claim compensation through our delay repay scheme.
"This only applies to season tickets which are valid for one month or longer. Flexi Ticket holders must have activated a day's worth of travel.
"You should make one claim for each day affected, claiming a delay of 120+ minutes, as this will provide you with the value of one day's travel.
"You need to apply for compensation online within 28 days of the intended travel date."
There was widespread travel disruption in the East of England due to hot weather on Monday afternoon.
On the Great Northern network, the railway "buckled" at Watlington in Norfolk.
"Services will be unable to run between Cambridge and King's Lynn," a company spokesperson said.
At London Luton Airport, a portion of the runway reportedly "melted" as temperatures exceeded 36C. EasyJet, Ryanair and WizzAir flights were temporarily suspended from 4.15pm until 6.05pm while engineers were on the scene.
In addition to Thameslink and Great Northern disruption on Tuesday, Network Rail has decided to shut the East Coast Main Line between London and York from 12pm until 8pm on Tuesday.
This means no trains will be able to travel through Stevenage, Peterborough or Doncaster until the closure is lifted, affecting several train operators, including LNER.
In the Hertfordshire, London Northwestern Railway will cancel all Abbey Line trains between Watford and St Albans, with all Marston Vale Line (Bedford - Bletchley) trains cancelled in Bedfordshire.
East Midlands Railway will run a limited number of main line trains. The firm has decided to cancel all trains on its Lincoln, Peterborough and Norwich (via Ely) routes.
At Greater Anglia, a spokesperson said a limited service is expected on most routes.
They added: "Passengers are warned to expect widespread disruption and short-notice cancellations, especially from the middle of the day onwards when the hottest temperatures occur."
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