This is causing significant delays and may result in service alterations and cancellations.
A Southern spokesperson said: "The particular type of fault is known as an 'axle counter failure
"Axle counters form part of the signalling system and allow the signaller to know where trains are."
Response staff are en-route and are expected to arrive at the site by 12.30pm.
Passengers are asked to allow plenty of extra time for your journey and check before you travel.