Members of the Communication Workers Union will walk out at 11am on 19 October after voting overwhelmingly in favour of 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.”
The Royal Mail said it was committed to hold talks with the union as a matter of urgency but said it will use all legal options at its disposal, including applying to the High Court for an injunction to prevent the action going ahead.
Mr Ward said: “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 said it was “very disappointed” at the announcement, adding: “There are no grounds for industrial action. We want to reach agreement.
“Royal Mail will be writing to the CWU invoking the external mediation process under the dispute resolution procedures in the Agenda for Growth. These legally-binding dispute resolution procedures were set up as a vehicle to resolve industrial disputes. We wish to use them to do just that.
“In 2013, Royal Mail and the CWU committed to the Agenda for Growth – a legally binding agreement. This agreement contains contractual dispute resolution procedures.
“Both sides are required to follow them once instigated. They escalate to independent external mediation, which we expect will take close to Christmas to be completed, and may be longer. External mediation as set out in the Agenda for Growth agreement has not yet taken place.”
It will be the first strike at Royal Mail since the postal group was privatised. The CWU said its members in Parcelforce will be involved in the stoppage, which will be the first national postal strike since 2013.
A Royal Mail spokesman added: “We believe any strike action before the dispute resolution procedures have been followed would be unlawful strike action.”