A runaway train went through six stops on a 13-minute journey on the Tube today with other Underground trains having to be cleared out of its path.
The unmanned engineering train, used for maintenance, began to move southbound from Archway station on the Northern line.
London Underground (LU) staff took an operational decision to let the train continue and diverted trains on to another branch of the Northern line while clearing trains from the Charing Cross branch on which the runaway train had been diverted.
The train eventually came to a stand at Warren Street station. LU and the Rail Accident Investigation Bureau have launched an investigation into the incident and LU has suspended the use of all engineering trains of this type.
LU said the train had become defective at 5.25am today as it approached Archway. Engineers began to move the train northbound by coupling it to an out-of-service Northern line train.
LU went on: "However, for reasons that are now under investigation, at around 6.44am the engineering train became detached from the Northern line train and began to move southbound.
"LU staff undertook a swift assessment of the incident and an operational decision was made to allow the train to continue, until it came to a stand at Warren Street on the Charing Cross branch of the Northern line, which it did at 6.57am."
LU continued: "Throughout the incident, LU staff monitored the situation, including the position of passenger trains. LU staff moved to divert passenger trains to the City branch, and direct the engineering train to the Charing Cross branch where passenger trains had been cleared. Services on the Charing Cross branch of the Northern line were then suspended.
The incident involved the suspension of the Northern line between Finchley Central and Archway and Camden Town to Kennington via Charing Cross - a suspension that was still in place by late morning.
LU director Richard Parry said: "Safety is our top priority, and we have of course launched an immediate and thorough investigation into this incident to establish the cause.
"Once that investigation is complete, we will publish the report, making its conclusions and recommendations clear. In the meantime we have prohibited the use of this design of engineering train on the Underground.
"From the start of this incident, an immediate assessment was made and operational decisions taken to minimise the safety risk to our customers and staff.
"Our engineers are working to restore full services as soon as possible, and we apologise for the inconvenience caused to our customers while we investigate this incident."
Pat Sikorski, assistant general secretary of the RMT transport union, said: "We are appalled and horrified at this major incident during passenger traffic hours and which could have very easily resulted in disaster.
"The runaway train, which it is suspected broke loose from a failed emergency coupling, represents a safety failure of the highest order. We understand that a collision with a passenger service train leaving Archway was only narrowly avoided."
Mr Sikorski said the RMT was "seeking urgent answers as to whether or not third party contractors were involved in this incident - particularly as the findings from Potters Bar (rail crash) ruled that third party contractors should not be involved in rail maintenance works".
He went on: "This horrific failure comes against a background of a systematic reduction in safety-critical jobs and safety procedures as a direct result of Transport for London's (TfL) financial cuts programme. In the light of this morning's events it is essential that TfL call a halt to their cuts plans and bring an end to the dilution of Tube safety."