A fault with the signalling system at Tulse Hill means all lines are blocked.
Trains may be cancelled, revised or delayed.
Disruption is expected until 00:45.
Customer advice:
Trains have been stopped in this area due to a major signalling fault.
Your journey is likely to be disrupted, and we're working to gather more information.
We don't have an exact estimate for any delays at the moment.
If this incident continues, it's likely that you may need to use another route, and we'll let you know more details shortly.
You'll be able to see any further updates on this page, so please continue to check back here.
Changes to services:
When we know more about changes to trains in this area, we'll update this part of the advice, so you can see how your train may be altered.
If you're waiting on a train which is delayed between stations:
If your train is delayed between stations, please remain where you are and listen to announcements, or speak to staff for further information. We'll get you moving as soon as possible. We're aware of your location, and our teams will be doing everything they can to help you continue your journey.
Please don't leave the train unless you are asked to do so. Other trains may still be moving in the area, and the track is also a very hazardous environment. In areas where the trains are powered by an electric third rail, this usually stays live and dangerous during incidents, and looks the same as other rails.
Ticket acceptance on alternative routes:
If needed, Southern and Thameslink will be working with other public transport companies to make sure that your ticket will be accepted on alternative routes.
If you can use another option for your journey, we'll let you know the details here.
Can you tell me more about this incident?
Your journey is likely to be disrupted by a complete failure of the signalling system in this area. The signals on the affected part of our network will have changed so that they are only showing red lights to drivers. This means that trains are unable to travel forward on any line. We won't be able to provide you with a train service at the moment.
The scale of this issue means that it could be a challenge to give you an estimate for the delays. We're working closely with Network Rail, who manage the signalling system, so that we can get more details. Their staff will be investigating as quickly as possible.
Complete signalling failures can be caused by a number of different problems - for example, power issues, cable faults or significant weather events. Sometimes there can be multiple faults which contribute to the same incident. As this type of failure is so disruptive, fault-finding and repair work will be given a high priority, and we'll pass on any updates. We'd like to thank you for your patience.
Check before you travel:
You can check your journey using the National Rail Enquiries real-time Journey Planner.
Compensation:
You may be entitled to compensation if you experience a delay in completing your journey today. Please keep your train ticket and make a note of your journey, as both will be required to support any claim.