Postal workers today complained they were being pressured into walking faster to complete their rounds under cost-saving measures.
The Communication Workers Union said delivery staff were being told to walk at a speed of 4mph, an increase over a previous target of 2.4mph.
The union said delivery workers were being pressed to complete "unrealistic" rounds to gain financial savings.
The Royal Mail denied the union's claims.
The row has flared over a software system that originated in Canada, which is now used by many countries including the UK to plan delivery walks.
The idea is that by using the system, deliveries will be more efficient, but the union said it had been introduced without agreement and claimed that a number of problems had broken out in various parts of the country.
A union official said today: "Royal Mail has altered the system to meet budgetary savings, not to consider the actual physical realities of delivery rounds.
"In some areas jobs are being lost or changed to part-time as the system finds efficiency savings. In other areas overtime is taken away.
"More and more managers are going out and delivering mail, which is not in their job description and they should not be doing.
"Mail backlogs or consistent returned mail, because of an over-estimated workload, have been reported this year in Watford, Enfield, Oxford, Coventry, Belfast, Cambridge and Evesham, amongst others.
"Managers in problem areas are putting delivery workers under undue pressure to complete unrealistic rounds. In some offices this is leading to bullying and harassment by managers trying to force delivery workers to take on larger rounds, work beyond their finish time and refusing to pay overtime. In extreme circumstances this is leading to suspension and even sacking."
The Royal Mail has denied that anyone was being bullied or harassed.Reuse content