Up to 136,000 postal workers are to be balloted on strike action after pay negotiations broke down.
The dispute involves plans to axe thousands of jobs
The Communication Workers Union (CWU) made the decision after it failed to reach an agreement with the Royal Mail.
The union said the move followed the imposition of a 2.9% pay rise and the failure to reach a deal on plans to axe thousands of jobs.
The Royal Mail said it was willing to hold talks but faced a huge challenge to modernise the business.
The CWU has said it is willing to resolve the dispute through negotiations and Royal Mail has responded by saying it was "very willing to talk with the union".
The union said it would send ballot papers out to its members by 10 July.
Speaking on the BBC, the CWU deputy general secretary Dave Ward said the delay was intended to allow time to reach agreement and "get a resolution".
However, Mr Ward also warned that the union was prepared to take strike action unless an "acceptable agreement" could be reached.
The Royal Mail has controversial modernisation plans that include issuing workers with a 20% share in the company.
But unions fear that this could mean 40,000 of the organisation's 165,000 workers losing their jobs, with many other workers forced to work part-time.
Mr Ward rejected what he called "phoney back door share schemes" that amounted to privatisation, saying the union wanted "higher pay now and in the future."
Meanwhile Royal Mail has said: "We have reiterated the company policy that any resulting job losses will be handled only through voluntary redundancy and natural turnover."