Weather forecasters have predicted record temperatures for the capital on Thursday with the mercury possibly hitting 100F (38c).
The current record was recorded at Heathrow Airport in 2015 with the temperature hitting 98.1F (36.7c).
The Met Office has issued a yellow weather warning as thunderstorms are to hit most of the UK, and the mercury could reach 97F (36c) on Wednesday.
Due to the high temperatures Network Rail and Southeastern have announced there will be speed restriction in place for Thursday.
Network Rail said in statement, “We may have to introduce speed restrictions during the hottest part of the day at vulnerable locations as slower trains exert lower forces on the track and reduce the likelihood of buckling.
“Passengers should check before they travel, by visiting their train operator’s website or National Rail Enquiries.”
Southeastern said they will also run a “significantly reduced service” due to the record temperatures.