More than 100,000 Royal Mail workers will vote on whether to strike over grievances about job security, an alleged culture of bullying, and the terms and conditions of their employment.
The Communication Workers Union (CWU) will begin balloting its members from 24 September, with the result expected in early October.
The union has said a culture of bullying in the workplace had not been tackled since the agreement, with local CWU officers being disciplined.
The deputy general secretary, Terry Pullinger, told a CWU meeting that members should get ready for the “fight of your lives”.
The CWU said Royal Mail bosses were “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 four pillars agreement was one of the final acts of the former chief executive, Moya Greene, covering pay rises, pension proposals and moves to reduce working hours from 39 to 35 a week by 2022, subject to productivity improvements.
The CWU said that labour relations have worsened since Greene was succeeded by Rico Back, who courted controversy when he picked up a £6m “golden hello” for taking over the company.
Walkouts are now running at more than one a week, including small, rural offices not noted for militancy, a CWU source said.
The union has also voiced concern about the company’s decision to move its Parcelforce business to a separate legal entity.
Parcelforce has previously attracted criticism over the employment conditions faced by drivers, some of whom claimed they were charged £250 a day if they were off sick.
The Guardian has approached Royal Mail for comment.