Post Office workers are to stage five days of strikes next week in a dispute over jobs, pensions and branch closures.
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."