The derailment of a passenger train was caused by a faulty set of points that had not been properly reset after maintenance work, it has emerged.
The front two carriages of the 0700 GNER service to Glasgow slipped off the tracks as the train left London's King's Cross station at 10mph on Tuesday morning.
The locomotive and a carriage of the train came off the rails despite attempts by the driver to stop when he spotted the defect.
Maintenance contractors Jarvis Rail had previously been carrying out overnight engineering work on the stretch of track where the derailment happened. The company is being investigated following the Potter's Bar train crash.
The company confirmed the points should have been left so trains would continue straight ahead but staff had "neglected" to reset them. It has started an investigation.
The carriages stayed upright and no-one was hurt but the incident has seriously affected parts of the UK's rail network.
Chief executive of Network Rail John Armitt said he was "furious" the derailment had overshadowed the opening of new Channel Tunnel rail link.
He said it was possible there had been a breakdown in communications between the Jarvis track workers and Network Rail signal staff.
They may have allowed trains to leave without apparently being aware of the problem with the points.
Platforms 1 to 6 were closed and an emergency timetable has been put in place.
Passengers travelling from Glasgow to London were being advised to transfer to Virgin services into Euston.
Those going from Edinburgh were told to go to London St Pancras via Sheffield.
GNER is running a train every hour, on the hour, from London to Edinburgh, and a train every hour, on the half hour, to Newcastle.
Journeys for passengers going between London and Leeds involve a change at Peterborough.
The revised timetable was expected to run throughout Tuesday.
John Bourn, spokesman for transport pressure group, Rail Future, said: "It is very unusual for a train to derail at such a low speed.
"This probably happened at the worst possible time and place for travellers."
Passengers are advised to ring National Rail Enquiries on 08457 48 49 50 or check on the National Rail website www.nationalrail.co.uk before setting out.