By Katie Law
BBC Radio 5 Live
Royal Mail managers are "bullying" workers into walking faster on their rounds, it has been claimed.
The Communication Workers Union (CWU) says that a new system, Pegasus, is being used requiring delivery staff to walk at four miles an hour.
Postal workers have told BBC Radio 5 Live that the pace set is "unachievable", and that some staff have been sacked for being too slow.
Royal Mail denies bullying and says staff should walk at 2.1 miles an hour.
A spokesman said the company did not tolerate bullying and would not ask its staff to do anything they were not capable of performing.
Postal workers say the problem centres on a Royal Mail computer system called Pegasus Europe Geo-route, which calculates the optimum post load that can be delivered by staff.
But they complain that the pace set is impossible to achieve.
The BBC was contacted by a listener who works as a postman for Royal Mail.
He asked that his name be withheld to avoid any possibility of identification.
The postman said: "The job is supposed to be done at a rate of four miles an hour.
"That is taking into account calling at doors for packets, recorded deliveries, registered letters.
"That pace is just not achievable."
To test out the postman's claims, BBC reporter Gavin Lee joined the postal worker on a round, which took four hours.
Gavin said afterwards: "The postman clearly felt under pressure to be as quick as possible.
"It was pouring with rain and we were both wearing thick winter clothing.
"I could barely keep up with him.
"After three-and-a-half hours a manager phoned the postman several times to see if he'd finished yet.
"There were several long driveways, and instead of walking to the doors, he ran.
"Every time he had to fill in any paperwork for a non-delivery he became agitated about wasting time."
Once the round was finished, the postal worker calculated what speed he had been moving at.
Measurements confirmed that he had moved at an average speed of 2.24 miles an hour - just over half the speed staff claim that managers demand.
The postman said: "That was working at the quickest pace I could do the job."
Our reporter said that the round he walked was a "light" one, but on some days it could take up to five hours to complete.
If he came back late or with unfinished post, the postman claimed he was bullied by his managers.
He says he is now being threatened with the sack because of the time he is taking.
He said: "You ring up and you tell the manager that you're really struggling, and he just asks: 'What's your problem?'
"It is psychological bullying."
Royal Mail denied forcing workers to 'speed up' during their rounds
The BBC spoke to more than 20 delivery staff who say they are being forced to work over their hours on a regular basis because the time allocated to complete their rounds is not sufficient.
Bob Gibson of the CWU, blamed the change in policy on Royal Mail, and said they were "going too far to make efficiency savings"
The union says two staff were recently sacked in Evesham for failing to complete their rounds on time.
It says it is being "inundated" with complaints from workers across the country about the speed they are being told to walk.
Royal Mail said it required its delivery workers to walk at 2.1 miles an hour, rather than the four miles an hour the union claims.
A spokesman said: "Pegasus is a highly effective computer aid to plan the most efficient way of delivering our customers' mail as quickly as possible.
"We have used it successfully in many hundreds of delivery offices nationwide since 1996.
"Royal Mail would never ask or require postmen and women to do anything they are not capable of performing.
"It is simply nonsense to suggest that we require anyone to deliver the mail at an average walking speed of more than four miles an hour.
"Royal Mail does not tolerate any form of harassment or bullying in any part of our business and we will take all necessary action to deal with any case that arises."