It comes as industrial action has swept across the country in various sectors over recent weeks.
Members of Unite backed walkouts by 86 percent and by 89 percent in Northern Ireland.
Royal Mail bosses plan to cut 542 frontline delivery managers' jobs alongside a redeployment programme to bring in worsening terms and conditions, the union said.
The dispute involves around 2,400 managers across more than 1,000 delivery offices.
Royal Mail managers have voted to strike
Andrew Milligan
Unite general secretary Sharon Graham said 'enough is enough'
Joe Giddens
Strike dates will be announced in the coming days and the union called for management to return to the negotiating table to reach a settlement to avoid disruption.
Unite general secretary Sharon Graham said: “It is no surprise at all that these workers have voted overwhelmingly for industrial action.
“Make no mistake, Royal Mail is awash with cash – there is no need whatsoever to sack workers, drive down pay or pursue this ill-thought-out redeployment programme.
“These plans are all about boardroom greed and profiteering and nothing whatsoever to do with securing this vital public service.
“Shareholders have been seizing the Royal Mail profits, while our members have been holding the service together. Enough is enough.
“Our Royal Mail members are guaranteed Unite’s 100 percent support in any industrial action they take this summer to get the company off this ruinous path.”