The strike, which is due to begin on 19 December, follows industrial action in October and September.
The union sought to place the blame for the strikes on the “intransigence” of the Post Office. But the company said its understanding was that talks were due to resume this week and said it was “extremely disappointed” by the decision to walk out.
Dave Ward, CWU general secretary, said members were 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.”
Andy Furey, the CWU’s assistant secretary, accused the Post Office of launching an “unprecedented attack on the jobs, job security and pensions of thousands of hard-working and loyal Post Office workers.”
He said staff wanted a pause in the Post Office’s closure and privatisation programme, and for it to hold off on its planned pensions changes. Furey said the union wanted to work with Post Office senior managers but accused them of choosing the “path of conflict and industrial disputes”.
The Post Office said it was “extremely disappointed” by the CWU’s action.