Staff at Exeter and Exmouth post offices will go on strike this Saturday as part of a nationwide row over jobs, pay and closures.
Members of the Communication Workers Union will walk out at 370 sites across the UK, including the Exeter and Exmouth Crown Offices, in a dispute over proposals to close or franchise out 70 of the branches.
The union said the plans would affect hundreds of jobs and be a further blow for already struggling High Streets.
Staff will be on strike at both Devon offices, in Exeter Princesshay and Exmouth Chapel Street, and picket lines will be mounted outside the post offices, with customers urged to back the union's campaign.
The Communication Workers Union (CWU) claimed that staff had not had a pay rise for two years.
"Our post office members are standing up against destructive plans which would slash 20 percent of the crown network and are simply asking for fair treatment and job security," CWU's deputy general secretary Dave Ward said.
"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," he added.
The union said the Post Office's Crown offices accounted for one-fifth of the business's total workload.
Crown offices are Post Office branches directly managed by the company - as opposed to locally-run by sub-post offices - mainly based in major High Streets.
Post Office network and sales director Kevin Gilliland said: "We regret any disruption to services the CWU's call for strike action may cause to customers.
"Crown branches are currently losing £40m per year and this is being subsidised by public money. This cannot continue."
