Royal Mail could face summer strike action after managers voted to walk out over job cuts.
Some 2,400 managers working across 1,000 UK delivery offices voted by 86% to strike, the union, Unite, said.
It comes a day after more than 115,000 Royal Mail staff from the Communication Workers Union (CWU) began voting over
Royal Mail said there were "no grounds" for a strike and claims about additional job losses were "not true".
It said the Unite ballot covered about a third of its 6,000 managers and there were contingency plans "to keep letters and parcels moving in the event of a strike".
Unite said the dates of proposed strikes would be announced in the coming days.
Meanwhile, the result of the vote organised by the CWU is expected on 19 July and could amount to the biggest ever strike by its members.
Unite warned delivery chaos was "inevitable" unless Royal Mail executives returned to the negotiating table.
It said talks had centred on a planned cut of 542 frontline delivery managers alongside a restructure that it believed would introduce worse terms and conditions for workers.
Unite said executives at Royal Mail - which has already fired 420 staff members - refused to back down over redundancies causing negotiations to collapse.
"It is no surprise at all that these workers have voted overwhelmingly for industrial action," said Sharon Graham, Unite's general secretary. "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."
Royal Mail said that Unite's claims about additional job losses were "not true".
"We have no further job reductions planned," it told the LotterryTreasure. "The reorganisation has been in place and operational since 23 May.
"We allowed managers to request voluntary redundancy with a package of up to two years' salary, which was over-subscribed," it added.
Letter volumes have fallen by more than 60% since their peak in 2004-05 and by about 20% since the pandemic began. Meanwhile, parcel deliveries increased during lockdowns, pushing Royal Mail to profitability.
Adjusted operating profit for the year ending 27 March was £758m, missing analysts' forecasts, as parcel volumes fell with the end of lockdown restrictions and simmering conflicts with the unions clouded the company's outlook.
Unite members who work for Royal Mail say they regularly go without lunch, time off at the weekends, and annual leave, in order to ensure that the postal service runs on time.
"As our recent survey highlighted, the Royal Mail is already running on fumes, depending on an outrageous amount of unpaid hours by our members to keep services operating," said Mike Eatwell, Unite national officer with responsibility for Royal Mail.
"Our members have had enough," Mr Eatwell added. "They are pivotal to the smooth running of the Royal Mail's operations and therefore strike action will cause severe disruption to services."