STAFF at a Tesco warehouse have to wear digital arm-band devices that constantly monitor their performance.
Workers at the distribution centre in Donabate in Dublin claim they got lower scores on the rating system if they keyed in that they went to the toilet or took a break.
Tesco said there was a 'break' function on the devices that was used to log stoppages, but denied it had any impact on productivity scores.
Known as arm-mounted terminals (AMTs), workers said that the Motorola devices were used to monitor the performance of 'order pickers', who load supplies, and forklift drivers.
They are not used by managers, administrative workers or security staff.
They said the device instructs employees how to pick their orders by scanning barcodes and stacking goods on a trolley.
It also has an in-built performance monitor, which grades them every time they collect goods in the warehouse and bring them to a dispatch area.
They said they got percentage scores for collection assignments, like loading beer or toilet rolls.
The devices give a set amount of time for a task, such as 20 minutes to load packets of soft drinks.
If they did it in 20 minutes, they would get 100pc, but would get 200pc if they were twice as fast.
One former picker, who did not want to be named, said staff were under huge pressure due to the devices, which are like Game Boys strapped to their wrists.
He said many of his colleagues were eastern Europeans, whose language skills were not good enough to get jobs in the services industry.
The worker said he got "surprisingly lower" scores if he took a break or went to the toilet. He said lunch break was not counted in the scores.
Sometimes, management would call staff to an office and tell them they had to do better if their scores were low.
"I had really easy assignments and when I'd come back after a break, I would get a horrendous score and wonder why," he said.
"One manager said to me that it was due to the breaks.
"You might get 80pc because you took a break, and would have to get 120pc later on to make it up. Some guys were amazing and got averages of 110pc all the time.
"The guys who made the scores were sweating buckets and throwing stuff around the place."
Tesco said the distribution centre at Donabate uses modern work practices, including the use of arm-mounted terminals.
It said the AMTs were a working aid and did not monitor staff on their breaks.
A spokesman said there was a 'break' function used to log stoppages for genuine reasons, including going on breaks.
"An average of 25 minutes a day has been factored into the system for genuine breaks," he said. "This has been in place since the system was introduced and means that break times do not impact on productivity scores in any way."
He said the "paperless order picking system" had been in place for five years and was a feature of any modern warehouse facility. The spokesman said Tesco had an agreement with SIPTU in relation to work practices.