More than 100,000 Royal Mail workers will be balloted for strikes if the company does not change plans to close its defined benefit pension scheme, a union has said.
The Communication Workers Union attacked the controversial decision to close the scheme next March and warned of a campaign of industrial action.
A private session of the union's annual conference in Bournemouth decided to press ahead with plans for a strike ballot unless the company changes its position.
Delegates condemned Royal Mail's announcement earlier this month, and approved a motion which read: "Conference recognises that Royal Mail's decision - along with their proposals - is a clear signal that they intend to attack our terms and conditions to maximise profit for the shareholder."
Deputy general secretary Terry Pullinger said the mood was one of "anger".
He added: "If we cannot make genuine progress and get an agreement by August, we will have to take a serious decision about balloting for industrial action."
The union has put forward an alternative proposal for a defined benefit "wage in retirement" scheme.
The conference decided that if Royal Mail introduces changes by executive action, there will be an immediate ballot for industrial action.