All Thameslink trains from Littlehampton to London have been cancelled for the rest of the day (February 6).
Network Rail warned damage to overhead electric wires between Harpenden and Beford is causing disruption between London St Pancras International and Bedford.
As a result, trains from Littlehampton to London have been removed.
Passengers are advised to use Southern services to Victoria and then the Underground to complete their journeys, according to Network Rail.
A spokesman for Network Rail said: “We apologise for any inconvenience this disruption may cause. We are working hard to resolve the issues as fast and as safely as possible. We thank you for your patience and understanding as we carry out this emergency work.”
A reduced service is also running from Bedford to London and Gatwick to London via Redhill.
Passengers travelling from East Grinstead to London are also advised to use Southern services to Victoria, and the underground.