Thousands of staff in the country's biggest post offices will go on strike on Saturday in a row over jobs, pay and closures.
Members of the Communication Workers Union (CWU) in around 370 so-called Crown offices will walk out in protest at plans to close or franchise 70 sites.
The union said hundreds of jobs will be affected by the proposals, which it warned could cause further problems for struggling high streets already suffering from shop closures. The CWU added that staff had not received a pay rise for two years.
The Post Office said Crown branches were losing £40 million a year and accused the union of ignoring the "harsh realities" the company faces.
Picket lines will be mounted outside the post offices, while customers will be urged to back the union's campaign.
Dave Ward, the CWU's deputy general secretary, said: "Our post office members are standing up against destructive plans which would slash 20% of the Crown network and are simply asking for fair treatment and job security.
"The Post Office's plans are short-sighted and would rob the network of the most productive offices while simultaneously putting hundreds of jobs at risk and potentially damaging local economies. We'd like to see a better vision for a successful network which maintains services in the heart of communities alongside quality jobs. We're confident this could be achieved if Post Office management would agree to negotiations."
Kevin Gilliland, network and sales director at the Post Office, said: "We regret any disruption to services the CWU's call for strike action may cause to customers. Crown branches are currently losing £40 million per year and this is being subsidised by public money. This cannot continue.
"The Post Office is transforming its network to improve customer experience and in turn bring in new business. We are committed to the Post Office remaining a key part of UK high streets and our plans ensure this will happen.
"They mean that for 70 of our Crown branches - less than 1% of our network - we will be looking to partner with a suitable retailer, whilst at the same time we will invest £70 million in the remaining 300 Crown branches to modernise and grow services to ensure their long-term viability."