Royal Mail workers have massively voted in favour of striking as part of a dispute about job security and employment terms and conditions.
Members of the Communication Workers Union (CWU) voted 97 per cent in favour of a strike - with a turnout of almost 76 per cent.
It is thought the workers could be preparing to take industrial action in the run up to Christmas.
The union says the Royal Mail hasn't stuck to an agreement that was reach last your covering a wide range of issues such as plans to reduce the working week and 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".
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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."
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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.
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