Nearly 1,000 postal workers at the country's largest mail depot are to be balloted over possible strike action.
The Communication Workers Union (CWU) is balloting staff at the South Midlands Mail Centre in Northampton.
The union said the dispute was over local job cuts, "unsanctioned budget savings targets and poor industrial relations".
A Royal Mail spokeswoman said: "All mail is being handled normally and services to customers are unaffected."
The centre, in Swan Valley, serves Northamptonshire, Milton Keynes, Coventry and Warwickshire.
The union said any walkout would cause "immense disruption" to the general public and businesses.
It said tensions began when local management refused to honour a local agreement on filling vacant roles at the centre.
Gareth Eales, area processing representative for the CWU, said: "Industrial relations have been deteriorating in our mail centre for the last six months or so, and the hard work and goodwill shown by the union and its members in opening the new mail centre has been traded for the pursuance of single rather than mutual interests.
"This disregard for employee morale and the potential threat to performance and service are arguably an insight into what a privatised Royal Mail focussed only on profit and not service would be like."
Royal Mail and the CWU are due to hold talks on Monday.