The issue was reported between London St Pancras International and Blackfriars at around 9.50am today (December 8).
Trains may be cancelled or delayed by up to 45 minutes with major disruption expected until 1pm.
National Rail has advised commuters to leave at least 30 minutes of extra time if you need to use an alternative route.
There will be a reduced service between St Albans City and Sutton and the service that would usually run through Mitcham Junction is suspended.
Trains between Bedford and Three Bridges through Redhill have also been suspended due to the repairs.
There is also major disruption to services from Liverpool Lime Street due a fault on a train at Edge Hill.
Major disruption is expected until 11pm.
