The RMT union has urged members to strike after a union activist was sacked for playing squash while off on sick leave with an ankle injury.
The driver was allegedly spotted playing the game "vigorously" by London Underground's revenue protection team.
The RMT will ballot its drivers on the Hammersmith and City Line of the London Underground (LU) in an attempt to reverse the decision, which they feel is backhanded and unacceptable.
The union claims the LU employee was not given a chance and that he has produced evidence that his GP advised him that increased exercise would speed up his recovery.
An LU spokesman said the man has been off work with this particular injury since May, and has spent a total of 218 days off sick since he joined LU in 1998.
"We are disappointed the RMT has chosen to order a strike ballot when the disciplinary process is not yet exhausted," he said.