Due to the extreme heat, no Thameslink trains will run on the Midland Main Line between Bedford and London Blackfriars (via Luton, St Albans, and St Pancras) on Tuesday, July 19.
This follows Network Rail's earlier decision to close the East Coast Main Line to and from King's Cross on Tuesday for safety reasons.
The route between Cambridge and King's Lynn is also expected to remain closed due to a buckled track in today's heat.
As a result, no Thameslink or Great Northern trains will run north of London on Tuesday, from London Blackfriars via St Pancras, or from London King's Cross or London Moorgate.
The closures are a precautionary measure because the heat expected on Tuesday, July 19, exceeds the safe design limits of the track and power systems on both the Midland Main Line and the East Coast Main Line.
Jenny Saunders, Customer Services Director for Thameslink and Great Northern, stated:
"There will be no Thameslink or Great Northern services north of London Blackfriars on Tuesday due to extreme temperatures having a severe impact on railway infrastructure."
"We apologise for any inconvenience this may cause, but the safety of our passengers will always be our top priority.
"Customers can use their Tuesday tickets on the following two days instead, or they can get a refund or compensation. Other than that, we strongly advise passengers to only travel by rail if it is absolutely necessary, particularly to destinations along the coast, where there is likely to be significant disruption."