Postal workers across Tayside and Fife could strike over plans to change their pension scheme.
Every Royal Mail depot across the region could close amid fears some workers would lose between £10,000 and £20,000 from their pension pots if the defined benefit pension scheme is closed.
Parcelforce workers would also be involved in the dispute.
The Communications Workers Union said it had made clear that any attempt by the company to impose change without agreement would be met with the “strongest possible opposition”, including a ballot for industrial action.
Royal Mail said that while the pension plan is currently in surplus, this is expected to run out next year.
It said there was “no affordable solution” to keeping the plan open in its current form.
A statement from the company read: “The company has come to the decision that the plan will close to future accrual on March 31 2018, subject to trustee approval.”
Kenny Logan, secretary of the CWU’s Scotland number two area, which includes Fife, claimed the Royal Mail had jumped the gun.
“We find the comments extraordinary,” he said.
“There is the possibility the CWU will apply for a national ballot which will include every Royal Mail employee.
“You are looking at processes, distribution and delivery as well as Parcelforce workers. It’s going to be massive.”
Mr Logan said that unless the Royal Mail withdrew its threat of disbanding the pension scheme in 2018, strike action was a real possibility.
That would involve between 1,500 and 2,000 workers in the KY postcode area.
But he added that he believed there would be a series of talks between Royal Mail and the union in a bid to avert a ballot.
“We believe Royal Mail have jumped the gun,” he said.
“Pressure has been put on them to make some sort of decision on what they intend doing with the pension fund and they have announced this without consent.”
Around 90,000 workers across the UK are in the scheme, to which Royal Mail makes contributions of around £400 million a year.
The company said if no changes were made, its contributions could more than double to more than £1 billion by 2018.
The CWU said that in place of the current defined benefit scheme, the plan was to put all members into inferior money purchase alternatives.
It has urged Royal Mail to withdraw its plans and enter into serious talks with the union.