Royal Mail workers will strike for 48 hours from noon on October 19 in a dispute over pensions, pay and jobs, the Communication Workers Union (CWU) said.
Members of the CWU voted massively in favour of industrial action, with 83% of those who voted backing the idea.
The CWU hope it is a "watershed" moment for unions as well as the Royal Mail, accusing the company of carrying out a "relentless" programme of cost-cutting to maximise short-term profits and shareholder returns.
The union says the company has closed its defined benefit, or final salary, pension scheme, offering new entrants an "inferior" scheme which the CWU claim will leave them in "pensioner poverty".
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."
Royal Mail has said it will use all legal options at its disposal, including applying to the High Court for an injunction, to prevent industrial action.
Royal Mail said: "We believe any strike action before the dispute resolution procedures have been followed would be unlawful strike action.
"Royal Mail is very disappointed by the CWU’s announcement and the company is committed to further talks as a matter of urgency to reach agreement with the CWU. There are no grounds for industrial action.
"These legally-binding dispute resolution procedures were set up as a vehicle to resolve industrial disputes. Royal Mail will use all legal options at its disposal, including applying to the High Court for an injunction to prevent industrial action."
This will be the first strike at Royal Mail since the postal group was privatised in 2015.