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.
Union workers say they have been "gagged"
The union said the offer was "premature", "not enough" and by imposing it "a blatantly hostile act".
Union members said the pay deal was not sufficient since it did not include overtime and other payments.
In a statement, Royal Mail said staff were entitled to the money, which would be backdated to April.
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 seem inevitable in the near future."
The latest disagreement comes as the Royal Mail and the CWU have clashed over new ownership plans.
Royal Mail recently unveiled plans to hand shares over to its staff.
Despite statements by the Royal Mail and the government that privatisation will not take place, union workers have opposed the scheme, fearing the staff shares move is the first step in that direction.
Union workers have alleged that they have been stopped from expressing their views on these new plans.
The union says that workers were "gagged" for expressing their views, and that posters calling for a ballot at work were removed by managers.
"For a public company to deny the opportunity of the workforce to express an opinion... is deeply disturbing for standards in public life," said the CWU's general secretary Billy Hayes.
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."