The defective track means a speed restriction is in place and, as a result, trains may be delayed by up to ten minutes or revised.
The disruption is expected to continue until 2pm.
National Rail said: “Reports of a track defect in the Pevensey area is affecting trains heading towards Eastbourne.
“Delays of between five and ten minutes are being experienced as trains run at lower speeds in the area as a precaution.
“To prevent congestion building up, some services will be run non-stop between Bexhill and Eastbourne.
“Network Rail staff are en route to the area to inspect and carry out any repairs required.”