Staff have been asked to sign agreements opting out of the European working hours directive, which limits staff to 48 hours a week.
Union chiefs from Amicus, which represents 14,000 post office staff, are threatening strike action after Royal Mail failed to rule out compulsory redundancies. The union has accused Royal Mail of exploiting a climate of fear and has said it is unfair for Royal Mail to ask staff to increase their hours after it made thousands redundant.
Peter Skyte, postal officer at Amicus, said: "The company has said 3,000 managers are surplus to requirements. This is clearly not the case if it is necessary for the remaining staff to work longer for the business to function. It is ludicrous for Royal Mail to want managers to do more than 48 hours a week while seeking redundancy from others."
Royal Mail has hit back and said that the request is purely a voluntary one and that staff are not being forced to work overtime.
The 3,000 jobs going are all non-operational management jobs and are part of the Royal Mail's three-year plan to reduce staff numbers by 30,000.
So far around 2,400 staff have taken up the offer of voluntary redundancy with the remaining 600 possibly facing compulsory redundancy. If compulsory redundancies are announced Amicus has said it will hold a strike ballot.
If the ballot of post office managers does go ahead it will be the first time in more than 25 years that they have been balloted.
"Amicus will now mobilise our representatives up and down the country to start the process of an industrial action ballot. This is the first time this has happened among our post office managers in more than 25 years, three Prime Ministers and more than 15 pieces of employment legislation," Skyte said.
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