Royal Mail workers are to strike on six days in December, including Christmas Eve, usually one of the busiest days of the year for the company.
The action is on top of four days already announced for late November, around the Black Friday shopping weekend.
The Communication Workers Union (CWU) said “the livelihoods of postal workers” were at stake.
Royal Mail said changes to the business were “not optional”.
The long-running dispute revolves around pay, jobs and conditions at the privately-owned firm.
The new strikes have been called for 9, 11, 14, 15, 23 and 24 December, the busiest time of the year for deliveries.
The CWU which represents more than 115,000 postal workers at Royal Mail, said it still wanted a negotiated settlement with Royal Mail Group and would “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,” it said.
Last month Royal Mail proposed a “pay-for-change” offer which would include changes to workers’ shift patterns including start times and Sunday working in exchange for a 9% pay rise spread over two years.
The CWU rejected the offer describing the company’s plans as the “Uberisation” of the postal service.
Royal Mail said: “Our preference is 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,” it said.