The Communication Workers Union (CWU) said it will ballot for a national postal strike this month if ongoing talks with Royal Mail do not lead to an agreement.
The union is in dispute regarding the settlement of its 2013 pay claim, further changes to the workers' pension scheme, the impact of privatisation on job security and workers' terms and conditions, and the company's future strategy.
Privatisation plans for the Royal Mail were first announced by the Government in 2011.
In July, Royal Mail proposed a three-year pay agreement for its workforce, including an 8.6% increase in basic pay over three years, which was rejected by the CWU.
All Royal Mail and Parcelforce workers who are members of the CWU will be balloted from September 20, with the result due on October 3. If there is a 'yes' vote, the union will give seven days' notice for strike action.
Meaningful and lasting
CWU general secretary Dave Ward said the union wants the Royal Mail to put in place protections that are "meaningful and lasting".
"We are looking to reach a groundbreaking agreement on terms and conditions that sets unprecedented legally binding protection for workers in the event of a sale, and regardless of who owns the company," he said.
"Postal workers know franchising, break up and sale of mail centres, distribution hubs and Parcelforce, along with the introduction of a new workforce on lower terms and conditions, are real threats in a race to the bottom with mail competitors for any new company."
A Government spokesman said industrial action is not necessary.
"It is disappointing that the CWU leadership has decided to ballot for strike action," the spokesperson said. "They are standing between their members and a generous pay offer of 8.6% over three years, which is more than teachers, nurses and our armed forces, who have had pay increases capped at 1%.
"Royal Mail management are continuing to talk to CWU and we encourage both sides to resolve this dispute."