Notice has been served on Royal Mail for a strike ballot among 110,000 workers which could lead to the first national walkout in a decade.
Members of the Communication Workers Union (CWU) will vote from next week with the result declared in mid-October.
The union is in dispute over issues including threats to the universal service obligation and claims of breaches of national agreements.
CWU deputy general secretary Terry Pullinger said "This union and its membership are in the fight of our lives. We face an assault on our terms, conditions and national agreements like we have never seen before.
"I am confident our members will stand with their union and deliver a massive yes vote in what is the most important dispute the union has ever had."
CWU general secretary Dave Ward added: "Our members, as always, will stand up and defend their jobs, futures and the service we provide to the people and businesses across the UK.
"Over the coming weeks we will be launching a significant media and public facing campaign and we will expose the actions of this employer."