Rail services on a route between Kent and East Sussex are being disrupted by a signalling system fault, a rail operator said.
Southern said services travelling between Ashford International and Hastings/Eastbourne are delayed by up to 30 minutes while engineers carry our repairs.
The incident was first reported by the company shortly after 07:00 GMT, stating the issue lay with track circuits used to detect trains.
It said some trains on the single-track route may run to a revised calling pattern, or terminate/restart at Hastings.