Tube drivers on the Piccadilly line are to strike on Christmas Eve and 5 January in a row over the demotion of a driver for passing red signal lights.
The strike could cause chaos for Christmas Eve shoppers
The workers at the Arnos Grove depot voted nine to one in favour of strikes on London Underground (LU).
Train drivers' union Aslef accused LU of unfair treatment of the driver.
Staff at the Acton Town depot of the District line voted to strike on 4 January in protest at "appalling facilities" at Earl's Court station.
The strike on the Piccadilly line on Christmas Eve will affect people heading to Heathrow Airport as well as Christmas shoppers.
Aslef claims an appeal hearing for the demoted driver took place without any union representation.
A spokeswoman for LU said: "Strike action will not resolve this issue."