The Communication Workers Union (CWU) will begin balloting its members from late September, unless an agreement can be reached before then, with the result expected on 8 October, according to the BBC.
Strikes at Royal Mail were averted last year after the postal service agreed to increase pay, reform pensions and reduce weekly working hours from 39 to 35 by 2022, subject to productivity improvements.
The CWU said this agreement is not being honoured but Royal Mail said it has awarded two pay rises since last year and cut the working week by an hour.
In a statement, the CWU said: “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.”
It added 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.”
Royal Mail said it was disappointed the union had set out a ballot timetable “while discussions are ongoing”.
A spokesperson added: “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 universal service.”