Britain's government was accused this week of introducing "slave labour" through a scheme that can see unemployed youngsters lose their benefits if they quit a work experience placement.
The ten-month-old voluntary programme, part of Prime Minister David Cameron's welfare reforms, is intended to help the record number of jobless youths improve their CVs and find work in a difficult economic climate.
But opponents say people who quit halfway through their placements are docked two weeks unemployment benefits as punishment -- and that the scheme allows major corporations to exploit a large pool of free labour.
The resulting furore has seen a number of big-name backers distance themselves from the scheme, including Tesco, the world's third-largest retailer, which was targeted by protests last weekend.
Campaigners say the 34,200 youths who have taken part in the scheme so far have provided unpaid labour worth £67.5 million ($107 million, 80 million euros) to companies that also include fast food giant McDonalds.
"If there are people who feel they're coerced to work for nothing, 'slave labour' isn't a bad description of it," Michael Bradley of the Right to Work campaign told AFP.
"If they aren't benefiting from doing these placements, and aren't getting jobs at the end of it, that's coercion."
Right to Work say many jobseekers fail to pick up new skills during placements that can last as long as two months, and instead spend up to 30 hours a week doing menial work for huge retailers.
"It's a demoralising experience for people to stack shelves, not get paid for it, and then know there's probably not a job coming out of it," Bradley said.
But employment minister Chris Grayling said critics were "job snobs" for sneering at careers in retail, which employs three million Britons according to the business ministry.
"There seems to be a view that a career in a supermarket is a bad thing," Grayling told BBC radio on Friday, adding that workers could rise from the shopfloor to management if they worked hard.
Protesters forced a London branch of Tesco to close last Saturday after the retailer posted an advert offering a permanent job in exchange for the weekly state-provided unemployment benefit of £53.45 ($85, 63 euros), plus expenses.
Tesco, the third-largest retail group in the world after US-based Wal-Mart and France's Carrefour, blamed a computer error for the advert, which it said had been "misunderstood".
It has since offered to pay jobseekers for four-week placements, guaranteed permanent positions for all "successful" participants, and urged the coalition government not to penalise youths who quit their placements.
But the government, under increasing pressure to tackle what Deputy Prime Minister Nick Clegg has called the "ticking time bomb" of youth unemployment, has defended its programme.
Ministers stress that the placements are voluntary and say they have helped find jobs for many of Britain's 1.04 million unemployed 16 to 24 year olds.
More than half of participants come off benefits within 13 weeks, the Department for Work and Pensions (DWP) said, and just 210 participants have lost their benefits as a result of quitting a placement.
"It remains a very popular scheme," a DWP spokesman told AFP. It is right to encourage young jobseekers to stick with the placements by penalising them for quitting, he added.
"It gives young people the mindset of: 'You have to go to work'," he said.