More than 100,000 Royal Mail workers are to be balloted for strikes over issues including job security and terms and conditions of employment.
Members of the Communication Workers Union (CWU) will vote from September 24, with the result due in the first week of October.
The dispute has flared over implementation of a so-called Four Pillars Agreement reached last year between the union and Royal Mail.
These include working hours, the use of technology to monitor efficiency and its parcel sorting systems.
“Although they are talking as if they are acting within that agreement I don’t believe there’s any true spirit and intent with them to support that agreement and that’s extremely concerning," Communication Workers Union deputy general secretary Terry Pullinger, said.
Royal Mail, however, insisted: “We are committed to open and constructive engagement with the CWU.
"We all want a successful and sustainable company that provides good quality jobs, fairness in workloads, and continues delivering."
The union said in a statement: "In setting this timetable we clearly believe that the business, in spite of their own media campaign, are not working within the spirit and intent of our national agreements and are following their own agenda that will have long-term detrimental effects on our members' terms and conditions of employment, job security and the future of Royal Mail Group as a whole."
The union has claimed that a culture of bullying in the workplace had not been tackled, with local CWU officers being disciplined.
Pullinger told a CWU meeting that members should get ready for the "fight of your lives".
