A signal system failure hit a busy Tube line today, causing severe delays in the morning rush-hour.
The problem, coming just over three weeks before the Underground starts coping with huge number of Olympics' travellers, happened on the Northern line.
Transport for London's (TfL) website warned passengers: "Severe delays are occurring while we fix a signal system failure."
Bob Crow, general secretary of the RMT transport union, said: "This is one of the worst Tube breakdowns this year and, just weeks from the Olympics, shows that the cuts to maintenance driven by TfL and (London Mayor) Boris Johnson have left services on a wing and a prayer with no plan B."
There were also minor delays on the Underground's Metropolitan line in the morning peak. This was due to a signal failure at Finchley Road.
London Underground operations director Nigel Holness said: "The Northern line has severe delays due to a power supply issue affecting the signalling system.
"Our engineers are on site investigating and working hard to fix the problem.
"We apologise to customers for the delays this has caused."