National Rail has warned of delays across the entire Southern rail network this morning.
A points failure at a maintenance depot near Selhurst prevented a number of trains from leaving the depot to begin operations for that day.
Engineers had repaired the points by 6.10am, and are now working to get trains out of the depot and into service as soon as they can.
"This late start is leading to a number of cancellations and alterations to services this morning. Some trains may also be delayed by up to 20 minutes," National Rail said.
There are also delays between Gatwick Airport and Redhill due to a signalling fault at Earlswood, meaning one of the two lines going towards London at Redhill is closed. Trains may be delayed by up to 15 minutes, cancelled, or revised.
"The Brighton mainline is one of the busiest areas on our network and with faults like these delays can quickly occur due to the volume of trains operating through Redhill," said National Rail.
Meanwhile, there are delays on London Overground services due to an electricity supply failure between Highbury & Islington and New Cross Gate earlier today.
Although the issue has been resolved, trains Between New Cross/Clapham Junction and Dalston Junction and between Crystal Palace/West Croydon and Highbury & Islington may be amended, cancelled or delayed by up to 10 minutes.