Royal Mail workers are to stage a 48-hour strike in a "watershed" dispute over pensions, pay and jobs.
Members of the Communication Workers Union will walk out at noon on October 19 after voting massively in favour of taking industrial action.
General secretary Dave Ward said: "This is a watershed dispute that will determine not only our members' pensions, jobs and pay but also the future of the service.
"Postal workers delivered a massive Yes vote for strike action and we are determined to take whatever steps are necessary to deliver an agreement that will protect and enhance our members' terms and conditions and improve the range of services on offer to customers.
"As well as this call to action, the CWU will now launch a major campaign to gain public and political support for postal workers.
"This is your service. We must work together to save it."
The union's deputy general secretary, Terry Pullinger, said: "It is highly disappointing that after 18 months of negotiations including external mediation that we have reached this point.
"Royal Mail Group management have clearly moved away from the spirit and intent of our agreements and the empty promises of privatisation, and have suffered a huge vote of no confidence from their employees and CWU members as a consequence.
"People are extremely angry and will now express that by overwhelmingly supporting this strike action in order to shift Royal Mail Group from their entrenched position, protect themselves and this great public service."
The union said the strike will involve around 110,000 workers across the UK.
The decision to press ahead with a strike followed an 89% vote for action in a ballot of members.
It will be the first strike at Royal Mail since the postal group was privatised.