From 3am on Monday 31st Octover, all 736 Crown Post Offices will be closed for a period of 24 hours.
The workers strike, which affects a 6,600-strong workforce in crown branches, back office and cash handling, is in protest against branch closures, job cuts and changes to pensions.
Further action closer to Christmas has not been ruled out.
Dave Ward, the general secretary of the Union said "The stakes have never been higher for the future of the Post Office, its workers and the communities they serve.
"The Post Office is at crisis point and the management and Government need to listen to the workforce.
"Staff and the public are seeing little more than a glorified closure programme from the Post Office and it cannot survive by simply cutting costs.
"We are calling for the Government as the owner of the Post Office to step in, halt the cuts and work with us to develop a proper strategy that will secure the future of the service."
It will be the second one-day strike over the dispute.