Services are unable to run between Lewes and Eastbourne which has led to cancellations for Brighton services to Eastbourne.
Southern Rail made the announcement just after 6pm.
The 6.13pm train from Brighton to Eastbourne has been cancelled, while the 6.20pm Eastbourne to Brighton service is also cancelled.
A spokesman for Southern Rail said: “We are aware of a failure of the electricity supply between Lewes and Eastbourne.
“No services are able to run from Lewes towards Eastbourne at present.”
It comes after power supply issues due to a damaged conductor rail just East of Lewes.
There have been engineering works planned all this week to fix the rail.