Drivers on one of the busiest Tube lines are threatening to strike in support of a colleague who was sacked for passing through a red signal.
The line is used by 660,000 people a day
The drivers' union Aslef said it would begin balloting members working on the Northern Line over possible industrial action in protest at the sacking.
The union said technical problems were to blame for the error and the result of an appeal was pending.
London Underground (LU) said the appeal process was ongoing.
Aslef said the driver, who had worked for LU for 22 years, followed the correct procedures when faced with the signal problem.
A spokesman said the driver was awaiting the results of an appeal which was heard on 18 December. He added that the strike would not go ahead if he was reinstated.
About 660,000 people travel along the line each day.