Passengers on the Thameslink Wimbledon loop service have been warned of a reduced service today because of flooding and damage to signalling equipment in East Sussex.
First Capital Connect, the service operator, said a reduced service would be in place all day on Monday, February 17.
There will be two trains per hour from Blackfriars towards Sutton and Sutton towards Blackfriars.
It comes after flooding near Patcham Tunnel, on the Brighton line in East Sussex, which has caused damage to nearby signalling equipment.
The operator said: "Passengers are advised that Thameslink services will be very busy and that they should seek alternative routes where possible.
"Tickets are being accepted on TfL services and by other trains operators.
"We are working closely with Network Rail to resolve the issues that are affecting our services."