Thousands of passengers have been urged not to travel to Britain's second busiest station on Thursday morning due to a signalling fault.
Dozens of Gatwick Express, Thameslink and Southern journeys to and from London Victoria were cancelled due to the fault in Streatham.
Network Rail, the Government-owned company responsible for maintaining rail infrastructure, advised passengers not to use those operators for travel from the south into the capital.
The problem with signalling equipment was discovered at 10.30pm on Wednesday and Network Rail engineers have been unable to fix it despite working overnight.
The cause of the fault has been traced to the failure of the power supply linked to signalling equipment.
Around 210,000 passengers use London Victoria each day. Southeastern services to the station appeared unaffected.
Rail bosses are attempting to arrange replacement buses to operate at "key locations", but these will be "subject to availability" as many are already in use to take children to school.
A Network Rail spokesman said: "Owing to a loss of signalling at Streatham Common, services are being disrupted to and from London Victoria.
"The signal loss means we are unable to run services between Balham, Selhurst and Haydons Road.
"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."
Thameslink passengers have already suffered weeks of disruption following the introduction of new timetables on May 20.