Royal Mail workers vote in favour of strike action in the run-up to Christmas
Members of the Communication Workers Union (CWU) backed action by 97% in a huge turnout of almost 76%.
The CWU said that Royal Mail is not sticking to an agreement reached last year covering a wide range of issues, including plans to reduce the working week, as well as job security.
Industrial relations at the company have worsened this year, with widespread unofficial strikes breaking out virtually every week.
Terry Pullinger, the CWU's deputy general secretary, said the union and its members were facing the "fight of our lives".
The CWU said the result represents the largest yes vote for national industrial action since the passing of the Trade union Act 2016.
The union said the prospect of the first national postal strike in a decade now "looms large".
Mr Pullinger said: "Just over one year ago the Royal Mail Group Board and the CWU agreed a blue print agreement for the future, a progressive agreement that included an historic pension solution, a mutual interest driven relationship and a joint vision for a successful postal service with social aims.
"Today the new RMG leadership are breaking that agreement.
"Our members take honour seriously and have voted to fight for that agreement against those who now seek to break up the great British Postal service in the interest of fast track profit and greed.
"Integrity and pride still matters and we will not stand aside and see what we have spent our working lives building destroyed."
General Secretary Dave Ward said: "This result sends a clear message to Royal Mail Group - our members will not stand by as you rip up their terms and conditions and destroy the service they give to the public and businesses of the UK.
"We would urge Royal Mail Group to now enter serious negotiations with this union. We also call on the public to get behind this dispute and your postal workers.
"We are very proud of our members today. They have stood by their union in record numbers and given hope to workers across the nation."
Mr Ward added that Royal Mail chief executive Rico Back should now consider his position following the "historic" vote.
The union's executive will decide the next move, but Mr Ward said: "We will look at what gives us the most leverage."
He said the union and its members were defending the postal service as well as their jobs.
A mediation process already in place ends on Friday, then there are three weeks before a "parting of the waves".
Mr Pullinger said what was happening to the Royal Mail was a "national disgrace", with an emphasis on profit.
"We represent decent, hard working people who don't deserve what is happening to them and the service.
"There is something seriously wrong with the way the Royal Mail is being run. When we make a deal we stick to it."
He claimed up to 50,000 jobs were at risk in the Royal Mail, as well as in Parcelforce under plans to separate it from the postal business.
He accused the chief executive of sacking managers and replacing them with "carpet baggers."
He added: "We are in a fight to the death.
"We built it - we would rather smash it to bits than hand it over."
Royal Mail said it was "very disappointed" that a ballot had been held.
A statement said: "A ballot result for industrial action does not necessarily mean there will be industrial action.
"We are still in mediation with the CWU.
"Under our dispute resolution procedure, set out in the Agenda for Growth, we are committed to reaching a resolution.
"No industrial action can be taken, and formal notification of industrial action cannot be given, before the conclusion of the Dispute Resolution Procedure.
"We want to reach agreement.
"There are no grounds for industrial action. Industrial action - or the threat of it - is damaging for our business and undermines the trust of our customers.
"Royal Mail wants to transform to meet our customers' changing needs as we post fewer letters and receive more parcels.
"Working together at pace, the transformation is about ensuring a more sustainable company, a fairer working environment, the best terms and conditions in our industry and a contemporary Universal Service."
Royal Mail said it was honouring the 2018 agreement, awarding two pay increases of 5% and 2%, granting the first hour's reduction in a shorter working week, and joining the CWU to lobby Government for a new pension scheme.
Postal Affairs Minister Kelly Tolhurst said: "I expect both parties to continue to engage in mediation talks to avoid industrial action - talking is the best way to resolve this dispute amicably.
"While this is for Royal Mail and the Communication Workers Union to resolve and come to an agreement, we are monitoring the situation closely and I stand ready to assist in any way I can."
Royal Mail said it had reached a negotiated agreement with Unite on a two-year pay deal for junior and middle-managers.
Under the proposals, managers will receive a pay increase of 2.6% this year, backdated to September 1, and a pay increase of 2.7% from September 1, 2020.
Over two years, this means a pay increase of 5.37%. Managers will also receive a £1,000 bonus advance in December 2019.
A ballot on the offer will now be held, said Royal Mail.