Lorry drivers have said Aldi and Lidl force them to do the jobs of warehouse staff as part of efforts to cut prices.
Their union said the practice should stop as it's not safe.
But the discounters said drivers are trained and insured and some like to get back on the road quickly.
Drivers told BBC Radio 4's You and Yours programme unloading deliveries is known as self-tip. Some complain they aren't being paid any more for this and the training isn't good enough.
The rise of the discount supermarkets has seemed unstoppable in recent years.
The latest figures from Kantar Worldpanel suggest that between them Aldi and Lidl now have more than 10% of all grocery sales - that's £1 in every £10 spent on food.
Lorry driver David Janczak-Hogarth says that exploiting drivers is part of the discounters' business model.
"It was obvious to me the only reason you were doing it was for the benefit of whichever discount supermarket it was that you were visiting. And you forego quality control by letting any Tom, Dick or Harry in your warehouse to unload their vehicle," he said.
Aldi told You & Yours that this is one of the ways it keeps prices low.
The discounter said: "We operate an efficient business model and pass on savings to customers who benefit from the lowest grocery prices in the UK.
"The majority of hauliers support this process as it saves them time and money. It means they can quickly get back on the road after unloading."Image copyright Getty Images
But other drivers told You and Yours they weren't happy.
One, who didn't want to be named, said of delivery to discounter Lidl: "I turned up and was told I had to tip myself. I said no I ain't. I've just driven four and a half hours to get here. So they sent an agency driver instead with the load and he got paid more than me. Since then I've just had to do it."
Lidl said it had a policy of drivers unloading their own vehicles: "As a retailer, we are not unique in this approach, which has also been verified by visiting regulators."
Both discounters said drivers are insured, get protective clothing and full training which should take half an hour. But drivers claimed the training can be as brief as ten minutes.
Big stores like Waitrose, Sainsburys and Asda said they do not allow drivers to unload, considering it safer for warehouse staff to do so.
Adrian Jones from the Unite union called on Aldi and Lidl to stop the practice: "A couple of years ago a driver died while unloading a delivery, not Aldi and Lidl but it shows how dangerous this can be. Things can happen when a professional driver is asked to do a job outside of their remit."