Commuters are facing delays of up to one hour this morning as railway signalling has failed.
An electrical power failure earlier this morning between Three Bridges and Gatwick Airport has had knock on effects for all major services.
Southern, Gatwick Express, First Great Western and First Capital Connect are all reporting disruption to their trains.
The routes are now open - but delays are expected as services clear the backlog.
A spokesman for Southern said: " Southern services on all routes via Gatwick and Horsham will be subject to delays of up to 60 minutes this morning owing to an earlier signaling failure which was caused by an electrical power failure.
"All routes are now open however service disruption is expected until further notice.
"Tickets will be accepted on Metrobus routes, Southeastern and South West train services and on TFL bus route 405.
First Capital Connect services between Brighton and St Pancras International are expected knock-on delays of up to 15 minutes, cancelations and supplemental bus services.
Gatwick Express has suspended services between Victoria and Gatwick Airport.
A spokesman for First Great Western said: "Disruption on First Great Western between Gatwick Airport and Redhill, and between Gatwick Airport and Reading due to earlier signalling problem between Three Bridges and Gatwick Airport.
"All lines have now-reopened but knock-on delays of up to 60 minutes and cancellations are expected.
"Buses are supplementing trains between Barnham and Three Bridges via Horsham. First Capital Connect, Gatwick Express and Southern are also affected."