Stores including Tesco and Argos have expressed concerns over a government work experience scheme which has been derided by critics as "slave labour".
The "sector-based work academy scheme" lets those on unemployment benefit work for a period without losing payments.
But Argos says it wants assurances that young people who do not take part will keep their benefits and Tesco has offered to pay those on placements.
However, the government insists the scheme is an "excellent" opportunity.
The comments come on a day when the government has launched a separate £126m scheme which gives companies cash incentives to take on and train more teenagers with poor qualifications.
The number of 16 to 24-year-olds who are out of work has risen above one million for the first time, leading to concerns they could become a "lost generation" in employment terms.
The sector-based programme offers the long-term unemployed work experience or training, while providing financial incentives to employers.
If jobseekers choose to take part but then fail to turn up without good reason their benefits could be removed. This has led critics to question whether the placements are really "voluntary".
The scheme has attracted adverse publicity recently, with opponents claiming large companies are using it for cheap labour.
Tesco posted a job advert looking for permanent night-shift workers at its branch in Bury St Edmonds, Suffolk, in exchange for jobseeker's allowance "plus expenses".
However, it removed this from the Jobseekers' Plus website after an outcry. The company said the advert had been placed due to an IT error.
Sainsbury's, and several other firms, are now not taking part in the scheme.
And, on Tuesday, Tesco wrote to the Department of Work and Pensions suggesting that, to avoid any misunderstanding about the voluntary nature of the scheme, the risk of losing benefits that currently exists should be removed.
The company has also announced that from now on any young person accepted for work experience will be offered an alternative. Under this they can be paid by Tesco for a four-week placement, with a guaranteed permanent job at the end of it, provided they perform satisfactorily.
Tesco UK's chief executive Richard Brasher said: "We know it is difficult for young people to give up benefits for a short-term placement with no permanent job at the end of it.
"So this guarantee that a job will be available provided the placement is completed satisfactorily, should be a major confidence boost for young people wanting to enter work on a permanent basis."
In a statement, Argos said: "We are very passionate about giving young people real opportunities to improve their employability and have worked over a number of years with job centres throughout the country on a range of programmes, but always with the objective of creating a meaningful role within Argos.
"We are in discussion with the Department of Work and Pensions to ensure the scheme is voluntary, meeting the work experience needs of the individual, and will keep this dialogue going to ensure no one is disadvantaged by working on this programme."
A Department for Work and Pensions source told the BBC two other large employers had expressed concern over the possible removal of benefits for those who do not take part in the scheme.
But Deputy Prime Minister Nick Clegg said it was "ridiculous" to condemn the government's efforts.
He added: "I think anyone who wants to condemn a scheme that helps people into work at a time of high unemployment really needs to think hard about their priorities. It is not slave labour. It is not compulsory. It is entirely voluntary."
Mr Clegg added: "It is very simple. We say to employers, 'Please take on these young people. We will pay them, through benefits, but could you please keep them on for a few weeks because it increases their chance of finding work'.
"Fifty per cent of youngsters on the work experience scheme so far have found permanent work. That is something that I celebrate. Other people might choose to condemn it. I don't."
Asked if he had any concerns about young people being asked to work a night shift stacking shelves in a supermarket for free, Mr Clegg said: "I have absolutely no qualms at all about the idea that rather than have a young person sitting at home, feeling cut off, lonely and getting depressed because they don't know what to do with their lives.
"It is better to give them the opportunity for a few weeks to actually work, and of course retain their payment through their benefits."
A Department for Work and Pensions spokesman said: "We have an excellent scheme that we know is making a real difference to the job prospects of young people.
"Tesco have said that they are continuing to be a part of the government's work experience scheme. What they have also said is that there will be delivering an additional offer to young people that will help more people find permanent employment. That has to be a good thing."