Members of the Communication Workers Union (CWU) backed action by 97% in a huge turnout of almost 76%.
The union claims that Royal Mail bosses are not sticking to an agreement reached last year covering a wide range of issues, including plans to reduce the length of the working week, as well as job security.
Although at this stage it is not clear when any possible industrial action will take place, the vote does raise the possibility of strikes being taken in the run up to Christmas.
"Thousands of posties have voted yes in record numbers for strike action in defence of their jobs," he tweeted.
"Its sell off led to shareholders creaming profits off the top while running down the service."
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 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
In a statement, the Royal Mail said it was "very disappointed" with the vote.
"A ballot result for industrial action does not necessarily mean there will be industrial action. We are still in mediation with the CWU," said the Royal Mail.
It added: "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."