The rail operator said due to a track circuit failure at Haslemere, trains have to run at reduced speed on the line towards London Waterloo.
Train services running through Haslemere may be cancelled and will be delayed – disruption is expected until 5am tomorrow.
The Portsmouth/Havant to Waterloo route will be affected.
South Western Railway said: ‘Track circuits are the method that our signallers use in order to detect the presence of trains on the track. ‘Therefore, until this [issue] is rectified, trains have to come to a stop and seek verbal permission to move forwards past the signal.
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‘We are sorry for the disruption to your journey.’
The 9:08am Haslemere to Guildford train, due 9:26am, will be cancelled.