Members of the Communication Workers Union (CWU), which represents post office staff in Crown Post Offices around the country, will walkout in a bid to pressure Government to reverse plans to franchise out 76 Crown branches.
The union claims the franchise plans will lead to 1,500 job losses and will be detrimental to customer service.
Workers are also striking over a pay freeze for all Crown post office workers, which has that's been in place since April 2012.
A spokesperson for the CWU confirmed other areas of the Post Office received planned pay rises in 2012 and 2013.
CWU national official Andy Furey said the dispute is gaining public and political support, and the determination of CWU members is "unwavering".
"Tens of thousands of customers have signed petitions against closures and a dozen political bodies - including the Scottish parliament, Greater London Authority and local councils - have passed motions against the company's proposals," said Furey.
"In fact, we can't find anyone who is in favour of them."
The Post Office said it is disappointed at the strike action and said around 370 Crown branches are losing £37 million a year of public money and must be brought into profit.
"We must continue with our plans to turnaround the Crown network to ensure we keep these branches on high streets and in city centres across the UK," said Kevin Gilliland, Post Office Network and sales director.
"We remain open to discussions with the CWU on pay options which do not add to the current loss of public money."
The Post Office has confirmed there will be no compulsory redundancies and it will provide support to those colleagues working in potential franchise branches as they consider their options.
The Post Office is a state-owned company, which receives £1.3 billion of taxpayer subsidy. The Government is aiming to privatise it before the end of the year.
