Post Office workers and managers will stage a fresh strike on Saturday over a dispute about jobs, pensions and closures.
Members of the Communication Workers Union (CWU) and Unite will mount picket lines across the country on one of the busiest days of the year.
The unions are embroiled in a long-running row over the franchising of Crown Post Offices - larger branches usually located on high streets - in addition to job-losses and the closure of a final salary pension scheme.
Dave Ward, General Secretary of the CWU, said: "The Post Office is now at crisis point. It needs a new strategy as a matter of urgency."
Unite union officer Brian Scott added: "We believe that Saturday is the day when most people will be dispatching their cards and parcels to their relatives and friends abroad.
"We are taking this action because the management refuses to talk in a constructive manner about the pension scheme, which is currently in surplus to more than £143 million."
The Post Office told customers that "almost all" of its network of 11,600 branches will be open despite the strike.
Kevin Gilliland, Post Office network and sales director, said: "Action affects fewer than 300 of our branches and many of these will be open to maintain services for customers preparing for Christmas.
"It will also be business as usual in the rest of our network - over 97% of branches will be operating their usual Saturday service."