The Communication Workers Union (CWU) has accused the Royal Mail of trying to impose a 2.9% pay offer that had been rejected by union leaders.
The company’s main union said the offer was “premature”, and imposing it was “a blatantly hostile act”.
In a statement, Royal Mail said: “Our people are entitled to a pay increase and we are writing to tell them what it is.”
David Ward, the CWU’s deputy general secretary, said: “We are refusing to be pulled into the punch today, but with the Royal Mail’s prevailing attitude, a confrontation seems inevitable in the near future.”
The Royal Mail and the CWU have clashed over new ownership plans, with the firm unveiling plans to hand shares over to its staff.
Union workers have alleged that they have been “gagged” from expressing their views on these new plans.
But a Royal Mail spokesperson said: “Political lobbying in the workplace is not covered by the framework Royal Mail and the CWU have agreed to.”