Members of the Communication Workers Union in England, Wales, Northern Ireland and most of Scotland will walk out on Monday, followed by a strike in a handful of branches in Scotland on Tuesday.
It will be the 12th round of industrial action since Easter, while staff are also taking other forms of action including a sales ban on financial products and services.
The union is opposed to plans to franchise or close 75 Crown offices, the larger sites usually found on high streets.
Dave Ward, CWU deputy general secretary, said: "If the Post Office thinks this dispute will simply fade away, they are sadly mistaken. Our members are fiercely opposed to the company's plans to close and franchise offices, slash jobs and impose a pay freeze.
"The company's plans are to downgrade the network, reduce services to local communities and hit jobs in the network. There is no mandate for this course of action and customers across the country are appalled at the reckless attitude of the Post Office towards these public services.
"We have tried talking to the Post Office about costs and efficiencies, but this is a company which made £94 million profit last year and paid significant bonuses totalling £15.4m primarily to senior managers. This can't be paid for by cuts to frontline jobs and services, it's simply not right.
"Next week's strike and the continuing sales ban activity will continue into the Christmas period unless management see sense and negotiate a fair deal for Crown office staff."