Royal Mail managers are to be balloted on strike action in a dispute over pay.
Unite said almost 5,000 of its members will vote in the coming weeks about whether to launch a campaign of industrial action.
The union said the managers had already rejected a "paltry" 1.3% pay offer, while officials said they had concerns about cost-cutting at the privatised postal company.
Unite officer Brian Scott said: "Our members are disgusted and upset by the abject failure of Royal Mail to take the issue of pay seriously.
"The company's final pay offer has already been rejected by 95% of Unite members, but the management remains unwilling to sit down and find a way through this difficulty.
"The dispute is about the 1.3% pay offer for some employees, with a non-consolidated lump sum for others.
"But there are a number of other serious factors that are of great concern to Unite members. Royal Mail has made a commitment to City investors to cut costs by £500 million and it is Unite members that are bearing the brunt of that."
A Royal Mail spokesman said: "It is disappointing that Unite has chosen to ballot its members.
"Royal Mail has engaged Unite in serious discussion around its pay claim from September 2015. We maintain that we have put forward a fair and competitive offer, which recognises the efforts of our managers and compares well with current inflation levels.
"We want to reach agreement so that our managers receive their pay award as quickly as possible."