Southern Rail says a fault with the signalling system between Eastbourne and Hastings is causing disruption to services.
The firm is warning that trains may be subject to delays or cancellations, until 1.15pm at least.
A spokesman for Southern Rail said: “Network Rail confirms a technical fault with their signalling infrastructure in the Bexhill area.
“Your journey to and from Ashford International will be extended by up to 30 minutes. You should check before you travel if your journey is essential. Your Southern tickets can be used with Southeastern between Hastings and London.
“We’ve also requested for rail replacement buses to operate from Ashford International, Hastings and Bexhill.
“We thank you for your patience during this time and we apologise for any inconvenience this may cause to your journey today.”