Breaking news: Christmas mail chaos as Post Office staff announce five day walk-out
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.
CWU assistant secretary Andy Furey said: "All of the blame for this unfortunate turn of events is 100% down to the intransigence of the company, who have launched an unprecedented attack on the jobs, job security, and pensions of thousands of hard-working and loyal Post Office workers.
"Our members want the Post Office management to pause its closure and privatisation programme, hold off on its planned pensions changes, and commit to sitting down with us and with the other key stakeholders of this Great British institution and, together, construct a lasting vision.
"We want to work together to build a positive future for the Post Office, its workforce and, of course, the people who we serve.
"The CWU can be a constructive partner to work with, but sadly, the people currently running the company have, so far, chosen the path of conflict and industrial disputes."
CWU general secretary Dave Ward said: "Our members are being forced into fighting to save their jobs and this great institution from terminal decline.
"We didn't want to be in this position, but unless we stand up now, the Post Office as we know it will cease to exist. We are defending the very future of the Post Office in this country.
"We want a Post Office that works for everyone, for communities, for small and medium-sized businesses, and for the people who serve them - our hard-working members, but the people running the Post Office have no serious plan other than further closures and managed decline and we won't accept that.
"We will be making a firm proposal for meaningful talks to establish a vision for the future and, if the company respond to that positively, then this dispute can be avoided."
Kevin Gilliland, the Post Office's network and sales director, said: "Just today, we agreed with the CWU that we would resume talks, which have been ongoing throughout the summer, on Wednesday.
"We are extremely disappointed that they prefer to resort to calls for strike action and we will be reviewing our position in light of this development. Our focus must be on supporting our customers, who rely on us at Christmas more than ever.
"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.
"The Post Office plays a vital role in communities all over the UK and the changes we are making support our commitment to keeping these services widely available into the future.
"Our progress is clear - over the last four years, we've dramatically reduced our losses and need for Government subsidy, at the same time as modernising nearly 7,000 post offices, adding more than 200,000 extra opening hours each week, and becoming the largest UK retailer open on a Sunday."