The union has warned a national strike could take place
More than 12,000 postal workers are to strike on Friday in a row over jobs, pay and services, the Communication Workers Union has announced.
The 24-hour strike will affect cities ranging from Edinburgh to Plymouth.
The union has accused Royal Mail of cutting the pay of employees and reducing services.
Royal Mail condemned the action, saying the union was resisting modernisation and said 90% of its staff would still be working on Friday.
Dave Ward, the union's deputy general secretary, said: "There are serious and growing problems in the postal sector which urgently need resolving.
"We have renewed our offer of a three-month no-strike deal to Royal Mail in return for meaningful talks over modernisation.
"The current cuts, bullying managers and ever-increasing workloads on a shrinking workforce cannot continue.
"Pressure and stress is at breaking point for postal workers."
Workers based in London, Edinburgh, Bristol, Darlington, Stoke-on-Trent, Plymouth, Norfolk and Essex will be taking part in the strike.
Protest letters will be delivered to the Royal Mail's chief executive, Adam Crosier, and Business Secretary Lord Mandelson.
The union said it was receiving an "ever-growing" number of requests for industrial action from postal workers across the country.
It said 400 requests for ballots on strike action have already been made and Mr Ward warned that without progress the dispute would turn into a national strike.
Royal Mail said the CWU was objecting to changes its leaders had agreed to in 2007.
A spokeswoman said: "Strike action hurts both businesses and individuals and our customers will not understand how the CWU leadership can keep saying it backs modernisation while constantly resisting the introduction of new technology and more efficient working practices on the ground."
Business Secretary Lord Mandelson told the business and enterprise select committee last week that the union was "boycotting" agreed procedures for implementing changes.
"If we are going to sustain the letters delivery service, however, there is a crying need for modernisation of this business. Both management and unions have said they are up for this," he said.
"I wish they would find the agreement to implement this rather than dragging the Royal Mail and the changes needed into a frustrating and never-ending, tedious process of endless negotiation, from which insufficient change follows at too slow a pace."
Controversial plans to part-privatise Royal Mail we put on hold by the government in June.
The bill, opposed by many Labour MPs, was due to go before Parliament before the summer break, but Lord Mandelson said it would not happen until "later".