Train services are being cancelled or delayed due to severe flooding.
Signalling equipment has been inundated at Patcham Tunnel near Brighton, forcing First Capital Connect to cancel services on the line between Bedford and Brighton.
Waters are expected to continue to rise, meaning disruption will last all day.
The train operator is running two trains an hour northbound and two southbound from Elstree and Borehamwood station.
Remaining services are very busy and First Capital Connect is advising people to use alternative routes where possible.
Tickets are being accepted on TfL services and by other trains operators.
A spokesman for the company said: “We are working closely with Network Rail to resolve the issues that are affecting our services.
“Please continue to check the First Capital Connect website for more information and accept our apologies for this disruption.”
For up to date travel information, visit www.firstcapitalconnect.co.uk