Brighton commuters were told to avoid making a trip to London before 9.30am because of a 30-minute power outage between Gatwick and East Croydon at around 6.30am.
This was followed by three points failures at Gatwick, Haywards Heath and Hove, adding to the disruption across the network.
Southern Rail said trains across the whole Gatwick Express, Southern and Thameslink networks may be cancelled, delayed by up to 60 minutes or revised.
It added that ‘technical difficulties’ across its apps and customer screens at stations could be showing incorrect information, and said station staff have been instructed to make additional announcements.
A Southern Rail spokesman said: “Earlier there was a national power outage for approximately 30 minutes in the Gatwick and East Croydon area. This had a major impact on Network Rail’s signalling system and our trains were at a stand across the whole network until the power was restored.
“In addition to this, there were also three points failures at Gatwick, Haywards Heath and Hove. These were rectified swiftly however Network Rail engineers will need to gain access to the railway line between Hove and Aldrington this evening. We will provide you with an update once we know more.
“Southern’s Control Teams have been working hard to restore trains back to the normal timetable in time for this evenings peak. However, when they experience severe disruption they often find themselves up against multiple challenges, a large number of our trains and crew have been left out of position for their normal journeys causing knock-on delays into the evening.
“If you are travelling this evening you are strongly advised to check before you travel as journey times may be extended due to ongoing disruption. You are also advised to check last trains and connections as these may be affected.”