Train bosses are asking people to only travel tomorrow and Thursday if absolutely necessary because of the heat.
Thameslink and Southern said that there would be significant disruption because of a reduced timetable and slower speeds to reduce the risk if any rails had buckled in the heat.
The Met Office has issued a rare red extreme heat warning for much of the south east. The south coast is currently subject to an amber warning but temperatures are still forecast to hit the mid-30s.
The Gatwick Express will be suspended from early afternoon on both days but Thameslink and Southern trains will still run, subject to further short-notice changes and cancellations.
Services on Friday are being kept under review as the forecast develops. Passengers should check before they travel.
Jenny Saunders, customer service director at Gatwick Express, Southern and Thameslink, said: “I’m sorry for the disruption this is going to cause but, with such extreme temperatures forecast, we have to run fewer trains, more slowly to keep services safe.
“I’d really urge our customers to think about what’s best for them and to travel only if absolutely necessary on Wednesday and Thursday.
“If you do have to catch a train, then consider travelling earlier or later in the day when it is cooler, or delaying your trip to later in the week.
“Please bring lots of water, wear sunscreen and a hat and have a back up plan in case of disruption.
“And please do check before you travel on one of our websites as the situation could change along with the forecast.”
Network Rail deputy chief executive Jake Kelly said: “Extreme heat can have a significant impact on the railway, so safety must come first.
“We’re asking passengers to check before travelling on Tuesday and only travel if absolutely essential on Wednesday and Thursday if they are going to, from or within the red warning zone as temperatures are expected to peak.
“If you do need to travel in this area or outside of it where it will still be hot, please plan ahead, allow extra time and take necessary precautions, such as carrying water, to stay safe. We thank passengers for their patience and understanding during this period.”
People who no longer wish to travel will be eligible for a full refund from where they bought their ticket for Gatwick Express, Thameslink and Southern services dated Tuesday, Wednesday or Thursday (23, 24 or 25 June).
