Members of the Communications Workers Union voted overwhelmingly to strike yesterday. Their action could take place in two weeks' time.
The vote in favour of serving strike notice was 89.7 per cent, with 85 per cent of the eligible membership voting.
The dispute centres on the position of about 850 clerical and middle management staff, who, according to the union, have not received pay increases promised in exchange for changes in work practices and productivity implemented two years ago.
This is denied by An Post, whose spokesman said an offer had been made through the Labour Relations Commission which the company thought should have been considered seriously by the union.
The spokesman said the company considered the ballot to have been "premature and unnecessary", and regretted any disruption to the service to the public that might come about.
He said he hoped the company and the union would be back in the Labour Relations Commission before the strike notice expired.
He said the question of wage increases and savings in An Post had to be seen in the context of the Flynn report.
That report found An Post lost €12 million last year and the company was likely to lose €15 million this year.
In a statement the Communications Workers Union said it regretted that a ballot for industrial action was necessary "to prompt inclusive and substantive dialogue with company management".
A spokesman for the union told The Irish Times yesterday he hoped the two weeks would be used constructively by the management to resolve outstanding promises regarding pay and reward structures.
"The employees of An Post have facilitated considerable progress through the 'Transformation through Partnership' process, and the CWU has reached agreement with the company in some areas on major changes in work practices and pay and reward structures," the statement said.
"However, in the post offices division the CWU has made no progress despite two years of patient effort."