Workers in hundreds of Crown Post Offices are to stage a third round of strikes in a bitter row over jobs, pay and closures.
The Communication Workers Union said its members at over 370 of the offices, the larger branches usually based on high streets, will walk out for half a day from lunchtime next Monday, April 29.
The workers have already taken strike action on Easter Saturday and last Friday in protest at plans to close or franchise 76 offices, which the union said would affect over 800 jobs.
The union said it was also seeking a fair pay rise for staff who it added have had no increase since April 2011.
CWU national officer Andy Furey, speaking at the union's annual conference in Bournemouth, said: "The Post Office has admitted that it's got its sums wrong on the cost of our pay claim. They're out by over £6 million - effectively doubling the cost of what we've asked for.
"This calls into question the accuracy of other Post Office numbers, including the alleged size of the losses in the Crown network.
"We already know that head office costs are a significant burden on the profitability of Crown offices and this should be tackled rather than attacking customer-facing frontline staff.
"We can help the Post Office put its numbers right, but they must be prepared to find a mutually acceptable solution to pay and the future of the network. Otherwise strike action will continue."
The Post Office said the Crown network was losing £40 million a year.