As the Met Office warned of ‘exceptionally hot’ temperatures of 39 degrees Celsius the rail operator said it is strongly advising customers to avoid travel.
Southern said high track temperatures ‘may cause delays and cancellations’ to journeys across the whole Gatwick Express, Great Northern, Southern and Thameslink networks until the end of the day.
A spokesman for Southern added: “Expect trains to be delayed during their journey, please plan extra time for critical journeys.
“Speed restrictions are in place on some routes because of the high track temperatures.
“This may cause delays and cancellations to journeys across the whole Gatwick Express, Great Northern, Southern and Thameslink networks until the end of the day.”
The operator warned passengers to carry water and not to board trains if they feel unwell.
The spokesman added: “Trains will be busier than usual due to the reduced number of trains in operation.
“Trains that do run will have extended travel times and delays on route so please travel as early as possible.”
The firm has ticket acceptance in place to assist passengers travelling today.
For full details of revised routes and ticket acceptance see the Southern Rail website.