Post Office workers are to stage five days of strikes next week.
The Communication Workers Union said the action will include Christmas Eve.
The union is embroiled in a long-running dispute over job losses, the closure of a final salary pension scheme and the franchising of Crown Post Offices - the larger branches usually sited on high streets.
Kevin Gilliland, the Post Office's network and sales director said: "We want to reassure customers that if further strike action takes place next week at least 97% of our 11,600 branches will not be involved.
"It will be business as usual in almost all of our network, with over 50,000 Post Office people on hand to support customers as they make their preparations for Christmas."