Police and ambulance crews were at the scene and services on the Northern Line were suspended following the incident which took place at 10.10am. There were no reports of any injuries.
London Underground said that all 100 or so passengers on board the train had been led to safety along the track to Euston station, which was a distance of about 150ft. A spokesman said that the cause of the derailment was being investigated.
All services were suspended both north and southbound along the affected section of the Northern Line, although a southbound service via Bank was still operating.
London Underground said later that 118 people had been on board the derailed tube. And it confirmed that nobody had been injured.
A spokeswoman said that investigations were still going on as to the cause of the accident, as a result of which the Northern Line was unlikely to reopen between Camden and Moorgate - the City branch - until today.
However, the spokeswoman said that normal services had resumed on the Charing Cross branch line.