Posties are set to strike on six days next month, including on Christmas Eve, in a fresh wave of industrial action.
Workers at the Royal Mail will take action on six days between 9 and 24 December, in addition to the four days of strikes already called.
The Communication Workers Union announced the new dates on Thursday evening, as part of a long-running dispute over pay and conditions for posties.
A spokesperson for the union said: “The CWU want a negotiated settlement with Royal Mail Group and will continue to engage the company to that end.
“But those in charge of Royal Mail need to wake up and realise we won’t allow them to destroy the livelihoods of postal workers.”
A Royal Mail spokesperson said the group’s preference was for “an agreement with the CWU but the change we need is not optional.
“They should be focused on a resolution to this dispute for their members and the long-term health of the business, rather than damaging strike action.”