Assistant secretary of the Communication Workers Union Andy Furey said: "All of the blame for this unfortunate turn of events is 100 per cent 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".
The union is embroiled in a bitter row over job losses, the closure of a final salary pension scheme, and the closure of Crown Offices, with services transferred to firms such as WH Smith.
Despite this, the Post Office is reassuring customers that services will not be impacted at other branches in the area, and all Post Office branches will be open on December 21, 22, and 23.
The government-backed post office operator has said less than 3% of its outlets will be affected by the action, expected to start from 19 December.
Kevin Gilliland, group network and sales director for the Post Office, said: "We are disappointed that, whilst we've been holding discussions with the union throughout this week and have made it clear we remain open to further, formal talks at the most senior level, they are continuing to cause concern to customers during the festive season".
"It will be business as usual in nearly all of our network, with over 50,000 Post Office people on hand to support customers as they make their preparations for Christmas".
A Royal Mail spokesman said: "Royal Mail will be operating as normal next week". Royal Mail has informed the Post Office that it has robust contingency plans in place and expects that there will be little effect on the services it provides.
"Post Office Limited has over 11,000 branches which will continue to operate as normal".
"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 almost 7,000 post offices, adding more than 200,000 extra opening hours each week, and becoming the largest United Kingdom retailer open on a Sunday", he said.
He added that the Post Office plays a vital role in communities all over the United Kingdom and the changes it is making support its commitment to keeping these services widely available into the future.