Unite said that some 2,400 Royal Mail managers working across more than 1,000 delivery offices had voted by 86% for strike action and 91% for action short of a strike. 

In Northern Ireland the figures were 89% and 88%. 

Unite is set to announce strike dates over the coming days and called for the Royal Mail management to return to the negotiating table “to reach a settlement to avoid disruption”, describing the group as “awash with cash”.

The dispute stems from a reorganisation that became effective in May. Royal Mail said that employees who left the business after a reduction of around 700 managerial roles did so through an oversubscribed voluntary redundancy programme with a package of up to two years’ salary, and Unite had taken the “very unusual step” to ballot after the restructure was complete. 

A Royal Mail spokesperson said: "We are disappointed by the announcement that Unite/CMA members have voted in favour of both industrial action and industrial action short of a strike, also known as work to rule. Unite have stated they will be informing us in due course in relation to the terms of any industrial action.

“Throughout the ballot process, Unite head office has misled members about additional job losses. This is not true. Unite has ignored our request to correct these claims.”

Unite has also accused Royal Mail of undermining agreed existing pay arrangements and “failing to act on the ‘long hours’ culture prevalent in the company”.

National officer Mike Eatwell stated: “Our members have had enough. They are pivotal to the smooth running of the Royal Mail’s operations and therefore strike action will cause severe disruption to services.

“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. But they have had enough of poor treatment and not being listened to.”

Eatwell said senior management at Royal Mail had lost the support of their own managers “and need to reflect carefully on their next steps”. 

“They need to come back to the negotiating table with a set of constructive proposals, otherwise strike action will go-ahead causing chaos to letter and parcel deliveries across the UK.”

Royal Mail stated there were “no grounds” for industrial action and it had contingency plans in place. 

“The extended consultation on these changes concluded earlier this year, and the restructuring is complete. We committed to protecting pay for all managers who stay with Royal Mail, and the vast majority will see an increase in their earnings. We allowed managers to request voluntary redundancy with a package of up to two years' salary, which was over-subscribed. We also made several concessions during the process, which Unite declined.

“The ballot covers around a third of our 6,000 managers and we have contingency plans in place to keep letters and parcels moving in the event of a strike.”

Separately, the result of the CWU ballot that could involve a strike by more than 115,000 postal workers, will be revealed on 19 July. 

In its full-year results for the 52-week period ended 27 March, Royal Mail Group had sales of £12.71bn, up 0.6% on the £12.64bn it achieved a year earlier. This included sales from its overseas parcels business GLS.

Adjusted operating profits at the Royal Mail wing were up nearly 21% at £416m. 

At the time CEO Simon Thompson said: “… as we emerge from the pandemic, the need to accelerate the transformation of our business – particularly in delivery – has become more urgent. Our future is as a parcels business, so we need to adapt old ways of working designed for letters and do it much more quickly to a world increasingly dominated by parcels.”

Royal Mail's share price has fallen by nearly 50% since the start of the year.