The Rail, Maritime, and Transport Union has said workers on Southern Railway are to stage fresh strikes for three days from December 22 and three days from New Year's Eve.
The strikes take in Christmas Eve, when thousands of commuters will be returning from London to their family homes, and Monday January 2, the final bank holiday of the festive period.
This is the latest wave of strikes in the bitter row over changes to the role of conductors. The RMT also claims backdated holiday pay is being withheld from conductors due to the industrial action.
A Southern spokesman said: "Passengers have had to endure 15 days of strikes so far and seven more days to come.
"These strikes have caused misery and hardship to people's work and family lives.
"We feel we cannot make these payments to conductors, who are currently taking industrial action.
"The fact they have taken strike action makes the calculation of what is owed more complicated. We plan to pay them once the industrial action is over."