Overrunning engineering works between Lewes and Seaford and Eastbourne has resulted in all lines being blocked, according to Southern Rail.
According to a statement on their website, the railway operators expected a normal service to resume at midday.
In it, they said: "We have been advised by Network Rail that engineering works in the Lewes area are currently overrunning. Upon inspection of the site a defect has been found on the track which needs urgent rectification.
Engineers are currently en-route to site with heavy machinery to commence works to repair the track. Due to the logistics of moving this heavy machinery, works are not expected to commence until 9am. Further updates will be posted as and when we have them."
As a result, trains cannot run between Lewes and Eastbourne and Seaford.
Rail Replacement Buses are currently en-route to Lewes, Seaford and Eastbourne, and were expected to be in operation at 7.30am.
Railway tickets can be used in the following ways:
Mutual ticket acceptance between Southern, Thameslink, Gatwick Airport Express services
Brighton & Hove Buses between Brighton and Eastbourne
Southeastern services between Hastings and London
Commuters took to Twitter to vent their frustration. @jubava1 said: "The “service” you provide is unacceptable. commuting to work using #southernrail made my life/work miserable. I do believe even in a desert I’d find more reliable public transport.