Hundreds of commuters were stranded on a broken down Tube train for more than an hour in the morning rush hour.
Maintenance firm Tube Lines said all power on board failed and an investigation into the cause has begun.
Passengers got off the train via another after it stopped in a tunnel near Swiss Cottage station on the southbound Jubilee Line.
The Line stayed closed for three hours after the breakdown on Wednesday, London Underground (LU) said.
A Tube Lines spokesperson said: "Tube Lines very much regrets the discomfort and inconvenience caused to passengers this morning as a result of a stalled train on the Jubilee Line.
"At this time we cannot confirm the cause of failure. We are, however, conducting a full investigation."