Postal workers are to stage a one-day strike after voting 4-1 in favour of walkouts over issues linked to the controversial privatisation of Royal Mail.
Members of the Communication Workers Union (CWU) will stage a walkout on November 4 after backing industrial action by 78%.
The union said it will consider further action and announced an escalation, with a new ballot which will enable postal workers to boycott competitors' mail.
CWU deputy general secretary Dave Ward said: "Postal workers have spoken very clearly that they care about their jobs, terms and conditions far more than they care about shares.
"The stakes have become much higher for postal workers since privatisation, making this ballot more important than ever. Postal workers will not be the people who pay for the profits of private operators and faceless shareholders."
The union balloted around 115,000 of its members in the Royal Mail and Parcelforce, and reported a turnout of 63%.
Mr Ward said the question now was whether a privatised Royal Mail still wanted an agreement.
"We have offered the company a two-week period to reach an agreement and, having already had many hours of negotiation, this is achievable if there is a will. The clock is ticking for both sides and we need Royal Mail to work to reach agreement before this deadline.
"What we want is a ground-breaking, long-term, legally binding agreement that not only protects postal workers' job security, pay and pensions, but will also determine the strategy, principles and values of how the Royal Mail Group will operate as a private entity.
"This means there will be no further break-up of the company, no franchising of individual offices or delivery rounds, no introduction of a cheaper workforce on two-tier terms and conditions and no part-time industry.
"It will mean - regardless of who owns Royal Mail - this company will not be able to enter the race to the bottom and replicate the employment practices and service standards of their competitors."
The union announced plans for a second ballot, asking workers to support a boycott of rival companies' mail.
Mr Ward added: "On top of a strike, the union will seek a mandate from members to carry out the boycott of competitors' mail. In law, this needs a separate ballot and we will commence the procedures for this at the earliest opportunity.
"The boycott will be used to supplement strike action. Royal Mail needs to reach an agreement with us now and avoid the consequences of these two types of action from postal workers."
The union has accused the Government of "deliberately creating a scramble" for shares, leading to many private investors cashing in their allocation at a big profit in the past few days.
The share surge sparked further anger about public assets being sold too cheaply.
Mr Ward said the Yes vote had been achieved despite the offer of free shares to postal workers, the timetable of the sell-off being moved before the ballot was completed, and a £300 "bribe" for workers crossing a picket line in the event of a strike.
He said the union has been pressing for a legally binding agreement so that work is not outsourced or franchised or that new staff are employed on cheaper rates of pay.
"We don't want this to become a part-time industry, with a race to the bottom on pay, terms and conditions. The most important people in this debate are postal workers and we want to make sure they get the deal they deserve."
Business Secretary Vince Cable has been asked to give fresh evidence to a committee of MPs over the privatisation following an appearance last week, when he faced tough questioning about the sell-off.
Investment bank Lazard is also set to be questioned next month by the Business Select Committee over the pricing of Royal Mail and concerns that institutional investors were allowed to buy into the company too cheaply.
The Direct Marketing Association, which represents the advertising mail industry, accounting for £1 billion of Royal Mail's turnover, said the strike will have a "severe" financial impact on tens of thousands of companies, charities and others.
Executive director Chris Combemale said: " The UK economy is only just emerging from recession and we cannot afford any further impediment to its recovery. The build-up to Christmas is a critical period that typically accounts for a significant proportion of businesses' annual revenues and charities' donations.
"People who rely on Royal Mail to deliver billions of pounds of goods ordered online would have their festive season disrupted, and loss of trade would lead to job cuts in many companies across the economy.
"Commercial users account for the biggest percentage of Royal Mail's turnover. Any disruption to service would quickly lead businesses to take their custom elsewhere, which is an outcome that would not benefit the postal workers that CWU represents."
A strike on November 4 will take place just seven weeks before Christmas, hitting the start of the busy festive season - the busiest time of year for the postal company.
Royal Mail said it was "very disappointed" by the strike announcement, adding that a ny action, or the threat of disruption, was damaging to its business, especially in the run-up to Christmas.
A statement said: "Royal Mail will do all that we can to protect our business and minimise the impact of any industrial action on our customers' mail."
The company commented on the 63% turnout, saying it showed that 51% of CWU members had voted against a strike, or had abstained.
David Cameron's official spokesman said: "The Prime Minister's view is that he hopes that industrial action won't go ahead.
"It is not necessary. It should be talks and not strikes to resolve issues."