Commuters have been advised to avoid London Victoria railway station after a signal failure caused major disruption across south London.
Gatwick Express services are suspended while Southern and Thameslink trains are badly affected, National Rail said.
There has been a total loss of signalling power on three separate supplies in the Streatham Common area, Govia Thameslink Railway (GTR) said.
Disruption is expected until the end of the day, National Rail added.
Victoria is the second busiest station in Great Britain with 75.8 million passenger entries and exits, according to Network Rail figures in 2016-17.
Both Southern and Thameslink have advised passengers not to travel to the capital.
A spokesman from GTR, which operates Southern, Thameslink and Gatwick Express, said: “In the meantime passengers are urged not to travel from the south on our remaining services, which will be heavily overcrowded.”
Gatwick Airport said flight passengers should leave extra time for their journeys.
National Rail said the fault with the signalling system meant all lines were blocked through Streatham Common station causing disruption, cancellations and delays of up to 60 minutes until the end of the day’s services.
Passengers said trains were “dangerously” overcrowded as those with tickets to Victoria were allowed on board other services.
Elaine Hindal said her train from Harpenden to London Bridge was a “nightmare” while Jessica Edwards said stations were “mobbed”.
Ms Edwards, 34, said she paid hundreds of pounds extra a year to travel on the Gatwick Express from Hassocks to Victoria.
She said Thursday’s problems meant she was an hour late to work, adding: “I have had to get off at London Bridge and get a bus to Hackney. It’s taking ages.
“All the stations are mobbed, so it’s not even worth trying to get the Overground or Tube.”
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A National Rail spokesman said: “We have had engineers working on site overnight doing all they can to fix the problem.
“However, we anticipate that the signalling will not be up and running this morning.
“We apologise for the inconvenience caused by the fault and passengers are being advised not to travel with Thameslink, Southern or Gatwick Express from the south into London this morning.”
The issues have also affected Thameslink services between Luton and Wimbledon, which have been suspended until further notice.
The disruptions come as GTR is set to launch a special compensation scheme for people severely affected by a chaotic timetable change in May.
Southern, Thameslink and Gatwick Express tickets are valid for travel on the following services, via any reasonable route, including:
- Across other Southern, Thameslink and Gatwick Express trains
- South Western Railway between Portsmouth, Southampton, Wimbledon and London Waterloo
- London Underground to and from Wimbledon
- London Overground between Clapham Junction and Watford Junction / West Croydon
- London Northwestern Railway between Watford Junction, London Euston and Milton Keynes Central
- London buses
- Great Western Railway
- Metrobus to and from Gatwick Airport
- Brighton and Hove buses
- Southdown Buses between Oxted and East Grinstead
- Compass Buses in the Tunbridge Wells area
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