The Communication Workers Union (CWU) sent out ballot papers to 121,000 postal workers yesterday. The result of the ballot is expected to be known in early October.
Millions of items of post have already been delayed because of earlier localised action over Royal Mail's modernisation plans.
However, Dave Ward, CWU deputy general secretary, said: "Modernisation is crucial to the future success of Royal Mail, but the implementation of change must be agreed and it must bring with it modern pay and conditions.
"We believe that Royal Mail management has completely mishandled the current situation. Disruption is hurting small businesses and other consumers, but postal workers are suffering more than anyone in the current dispute. Small businesses stand to suffer more with reduced services in the future if Royal Mail doesn't reach a national agreement."
Royal Mail managing director Mark Higson condemned the action: "It beggars belief that CWU chief Dave Ward says today that the disruption caused by the CWU strike is hurting customers, yet at the same time calls a national strike ballot to step up the damage they are already inflicting on customers big and small.
"The CWU leadership is well aware that it has already agreed all the changes Royal Mail is making and we urge them to recognise the tough economic conditions faced by all our customers and Royal Mail itself, and to live up to their claims to support modernisation and to focus on delivering the postal service on which so many customers depend."
Earlier this month, the Direct Marketing Association called on the government to step in and resolve these issues without disrupting postal services to its members. Robert Keitch, chief of membership and brand, said: "It is quite clear that businesses cannot sustain these interruptions indefinitely.
"Should relations deteriorate further and a national postal strike take effect, this would have a significant impact on businesses that rely on issuing invoices and receiving payment via post.
"We are seeing evidence that the economy might be staging a recovery from the recession and yet strike action could be the final blow for many struggling companies."
