Postal workers will launch further local strikes this week as industrial relations worsen before a national ballot for action.
It follows a series of local strikes last month over Royal Mail modernisation plans, which unions claim will amount to cutting pay and jobs.
The Communication Workers Union warned about industrial action earlier this year after it claimed the postal delivery organisation was pressing ahead with "arbitrary" job cuts.
The union's members now plan to walk out in the Midlands, London, Yorkshire, Bristol and Northern Ireland, the Guardian has reported.
The company has maintained that the union is refusing to co-operate on an agreement to modernise the service.