Britain's ranks of teenage dropouts could grow under plans to try and force them to continue in education or training, according to a study of disadvantaged youths.
Instead of eliminating the Neet group - students not in education, employment or training - the authors of Engaging Youth Enquiry warned that employers could be put off hiring young people because of the training costs.
The inquiry, by the Nuffield Review of 14 to 19 Education and the education charity Rathbone, concluded that forcing them to attend school or college was likely to fail.
Those out of work and not in education or training had typically been truants from compulsory education for many years, the 18-month investigation found.
Geoff Hayward, director of the Nuffield Review, said that contrary to the Leitch report - into how Britain's skills training equipped the country to compete globally - many of these youngsters were in low-skilled jobs. Research showed they were likely to remain so. But their employers could choose other workers rather than take on the cost and disruption of training.
"I suspect that many employers would want to do that," he said. "They will just substitute other forms of labour. It could lead to an increase in the number of Neets."
Richard Williams, chief executive of Rathbone, said: "The reality is that many of them have not been in school since at least the age of 12. They may have partial attendance, but they've got a substantial history of not engaging at school.
"There's a question of whether changes to the legal requirements will, in itself, have any impact on people who have a substantial history of not being in attendance."
Despite colleges' efforts to promote a more adult ethos and to widen participation, many of the teenagers interviewed regarded them as impersonal and autocratic - just like schools, as they saw it.
Mr Williams said: "There are many young people who will drop out of college, who have the same problems with college as they have with schools - they are large, often impersonal institutions."
Education maintenance allowances had been the biggest success in attracting people back to education, helping to reduce the numbers of Neets by one percentage point last year.
But they predict that the new diplomas will fail because they lack real practical work. Instead, it suggests that there should be a work-based route to qualifications that would be open to teenagers who are not yet ready for a full level two qualification (five good GCSEs), required for apprenticeships.