Postal workers across the UK could be set to strike this summer in a row over pay.
More than 115,000 workers will vote in the coming weeks over whether to take industrial action.
Notice of the ballot has been served to the Royal Mail by the the Communication Workers Union (CWU).
The union, which represents postal workers, has raised concerns over what they say is a real-terms wage cut of 9% for Royal Mail employees, with inflation set to soar.
Delegates at the union’s annual conference in April voted to declare the start of a national pay dispute in early May if a “straight, no strings” pay increase failed to materialise.
The Royal Mail has defended its response, having said it offered the “biggest increase” for many years.
CWU deputy general secretary (postal) Terry Pullinger praised members as “heroes” as he insisted the union will defend itself.
“Throughout this entire dispute, Royal Mail management have conducted themselves insultingly and disrespectfully to key workers,” he said.
“Their conduct, and particularly the imposition of such an aggressive pay offer, has eroded trust among loyal employees.
“Nobody wants to be in this situation, but our members are heroes.
“We will defend ourselves if provoked – and we are convinced we will receive our biggest ever Yes vote for action.”
CWU General Secretary Dave Ward insisted that the current state of affairs is “unjust and unsustainable”.
He said: “Our members have been treated in a completely undignified manner by the people they make incredible profits for.
“Our members kept this country connected throughout our country’s greatest crisis since World War Two, and many paid the ultimate sacrifice.
“Now, they are being told there’s nothing for them, and they have to accept having less while the mega profits of bosses come first.
“The state of affairs is unjust and unsustainable, and I have no doubt that our 115,000 members will stand strong against it and deliver a historic vote for action.”
A Royal Mail spokesperson rejected the suggestion that there is a need for action to be taken.
“We believe there are no grounds for industrial action. We offered a deal worth up to 5.5% for CWU grade colleagues, the biggest increase we have offered for many years, which was rejected by the CWU,” they said.
“We have further talks on change with CWU this week. We hope this will ultimately lead to an agreement on the changes required to ensure Royal Mail can grow and remain competitive in a fast-moving industry, securing jobs for the future and retaining our place as the industry leader on pay and terms and conditions.
“We value the work we do with CWU and remain committed to agreeing a deal for tomorrow, not just today.”