Some Royal Mail postal managers are reaching breaking point due to increasing work loads and staff shortages, a union has warned.
Managers are delivering mail due to staff shortages - Amicus
Amicus called for an urgent meeting with the postal group's chairman Alan Leighton over what it calls a crisis.
Amicus, the biggest private sector union, says it deals daily with cases of Royal Mail managers suffering stress and working long hours.
But Royal Mail said a recent survey showed 86% of its managers were happy.
Even more of its managers told the survey that they were proud to work for the firm and so it did not recognise the picture painted by the union.
However, Amicus says managers are being forced to deliver mail because of staff shortages.
The union released a copy of a letter from one London delivery manager and said the correspondence was an example of the enormous pressure that managers now faced.
In the letter, the manager begs senior staff to be downgraded even though it would result in a substantial pay cut.
He complains that his health has suffered, with bouts of crying for no apparent reason.
Up to 136,000 postal workers are to be balloted this month on strike action after pay negotiations broke down.
The Communication Workers Union (CWU) said the move followed the imposition of a 2.9% pay rise and the failure to reach a deal on plans to axe thousands of jobs.
Royal Mail has said it was willing to hold talks but faced a huge challenge to modernise the business.
In May, the company confirmed it was to receive a £1.75bn package from the government to help plug its pension fund deficit and pay for modernisation.