There are major delays to trains coming to and from Portsmouth this afternoon.
All London bound lines were blocked after reports of a track circuit failure just south of Woking Station.
National Rail said the signalling problem has been resolved but there may still be delays while services get back to normal.
Earlier South West Trains said services from Portsmouth to London Waterloo via Eastleigh would terminate at Winchester.
Direct services between Portsmouth and London Waterloo also ran via Cobham.
Disruption is estimated to last until about 4pm.
Track circuits are used by signallers to detect the presence of trains on the track.