Southern Rail said services between Brighton and Haywards Heath are delayed this afternoon (March 25) following a fault with the signalling system.
The rail operator said disruption is expected until at least 2pm.
The fault is in the Hassocks area, and will affect services from Brighton to London Victoria and London Bridge.
A Southern Rail spokesperson said: “Network Rail has confirmed a technical fault with their signalling system in the Hassocks area, affecting the line towards London Victoria and London Bridge.
“Whilst the failure is being investigated by Network Rail, a temporary work around known as ‘talking past’ has been implemented. This means that upon arrival towards the affected signal, train crew must stop and contact the signalling centre whereby the signaller will verbally guide the driver until the next signal is reached.
“This process will unfortunately add up to 10 minutes to you journey, however is the most suitable decision to ensure that a full as possible service can still run.”