Postal workers in Plymouth are set to stage a mass walkout of around 500 workers.
Six of the city's postcode area Royal Mail delivery offices could now be hit following the ballot.
The Communication Workers Union (CWU) said members had voted overwhelmingly in favour of the strike – in some cases unanimously.
The results, based on an average turnout of almost 70%, begin at 85% in favour with two units returning 100%.
Officials said the dispute, over national service agreements and allegations of bullying, will be the biggest in city history
Union representatives will now meet to agree the next steps with notice of any action expected to be given to Royal Mail within a week.
CWU divisional representative Chris Webb said: "We are seeing a postal service being run into the ground and shaped for privatisation by an aggressive employer. We will not stand by and tolerate this any longer."
If industrial action goes ahead, the local dispute in the Plymouth area will affect postcodes PL1-4, 5-6, 8-9, 19, 23 and 24-26.