Southern Rail and Thameslink have been hit by a signal failure as thousands of passengers face rush hour travel chaos.
A "major signal fault" at around 4am this morning resulted in disruption to networks as "no trains" were expected to run until further notice.
Passengers, including commuters and holidaymakers, were warned not to travel due to the fault.
Some services resumed at 7am but Gatwick Express trains continue not to run.
A Southern Railway train pulls into Three Bridges Railway Station
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Services between Littlehampton and London Victoria have also been cancelled.
Disruption is expected to last until midday but reduced services will also hit the service.
Reduced services are operating between Three Bridges and Brighton, and Brighton and Chichester.
Passengers on impacted routes have been asked to delay travel until later or allow "at least" an extra 60 minutes for journeys.
LATEST DEVELOPMENTS:Some stations will likely experience queuing systems and running services could witness more passengers than usual.
Southern Rail provided a list of alternative services for commuters, including bus services.
Thameslink has since resumed usual service on many of its lines.
However, trains continue to face disruption between Cambridge and Brighton.
However, passengers can use tickets to travel on Great Northern trains between Cambridge and London and any alternative Southern or Thameslink service from London to Brighton.
The service between London Bridge and Charlton was also disrupted, an update released at 8.30am confirmed.
Thameslink trains will currently not stop at Deptford, Greenwich, Maze Hill or Westcombe Park due to the disruption.
Major faults were mostly rectified by 7am but Southern Rail continued to deal with points failures at Haywards Heath, and between Preston Park and Hove.