Royal Mail workers have voted 4-1 in favour of strike action over pay and conditions and the privatisation of the 500-year-old postal service.
More than 100,000 postal workers will walk out on a 24-hour strike on 4 November unless the company agrees to the union's demands for improved pay and conditions.
The Communication Workers Union, which balloted 115,000 members, said 78% of those who voted had backed strike action, despite the company offering all staff a £300 bonus if they committed not to strike. Turnout was 63%.
The union said it was considering further action in the runup to Christmas if the dispute cannot be resolved.
Dave Ward, CWU deputy general secretary, 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.
"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 groundbreaking, 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."
The union also announced plans for a second ballot, asking workers to support a boycott of rival companies' mail.
"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," Ward said.
"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."
Michael Fallon, the business minister in charge of the privatisation, has attacked the union for pressing ahead with plans for strike action despite an offer of an 8.6% pay rise over three years. The CWU dismissed the pay offer as a misleading and unacceptable sweetener designed to win support for the privatisation.
Royal Mail said it was very disappointed that the CWU was pushing ahead with the strike ballot. The company said an Ipsos Mori survey it commissioned found 78% of Britons think the company's pay offer is fair or very fair, and 72% of the 1,468 people polled said the union should accept it and call off the strike.
Royal Mail's shares, which have risen by more than 40% since the privatisation on Friday, were down 3.4% to 473p following the strike announcement.
