Several Southeastern and Thameslink trains which service London Victoria and Blackfriars are facing disruption on Thursday morning because of a fault with the signalling system.
Multiple London stations are affected by the fault - between Bromley South and Denmark Hill.
Some Elizabeth Line and Greater Anglia services between Stratford and Shenfield are also facing delays.
Meanwhile, the Overground is part suspended and minor delays have hit the Hammersmith and City and Central Lines.
The Piccadilly Line is again part suspended between Cockfosters and Hype Park Corner, with sever delays on the rest of the line.
