Colleges hoping to teach 14-year-olds full-time run the risk recreating the divide between secondary moderns and grammar schools, a think-tank will claim.
A forthcoming book from Civitas, Unqualified Success: Investigating the State of Vocational Learning in the UK, is to argue that under-16s are increasingly being led towards "substandard" vocational qualifications that damage the reputation of vocational education and fail students.
Anastasia de Waal, the think-tank's education director and author of the book, said the greater likelihood of students from low-income families enrolling on such courses risked creating an "educational apartheid". Allowing colleges to recruit more under-16s would exacerbate the problem.
"It isn't too dramatic to liken it to grammar schools and secondary moderns," she said. "They're losing subjects that are going to make a huge difference to their lives - they need that general knowledge from history and geography.
"Thirteen-year-olds are making these decisions, but how are they going to know what area of work they want to go into - particularly when they are disengaged from education?"
Countries such as the Netherlands, which experimented with vocational programmes at 14, are now changing their minds, she said.
More than 100,000 Year 10 and 11 students spend part of their week in college. Research suggests vocational courses have exceeded their targets for encouraging students to stay in education. Four thousand 14- to 16- year-olds study full-time in college, under agreements with local authorities.
The Association of Colleges (AoC) has long argued for changes allowing more students to study full-time. The Liberal Democrats backed these in their election manifesto.
AoC policy director Lesley Davies said colleges recruiting at 14 would be able to offer a broad and varied curriculum that avoids trapping them into a narrow pathway.
"People at 13 already make important decisions about which courses they take," she said. "The pressure is always on to make early decisions because so much depends on it. But people change their minds at 14, 15 and 16, and colleges can accommodate it - I'm in my 50s and I'm still changing my mind about what I want to do."
Other Government-backed initiatives suggest a choice between vocational and academic routes at 14 is gaining momentum. Lord Baker, promoting 14-19 University Technical Colleges, argued that a split at 11 was too young, but that students volunteering for practical courses at 14 were likely to thrive.
Among the qualifications Civitas will criticise are the BTEC Firsts and OCR Nationals, "vocationally related" GCSE equivalents that the think-tank says do not equip students for a career but test limited knowledge, such as how to make drinks for hospitality courses.
Sixth-form colleges rate the qualifications at about half of their official value, the book will claim, and employers do not value them. Ms de Waal quoted Herbert Berger, Michelin-starred chef, saying: "They are not equipped to perform in the industry."
A spokeswoman for Pearson, owner of the BTEC awarding body Edexcel, said: "We absolutely stand by the rigour of the BTEC vocational qualifications we offer, which develop not only practical skills but knowledge that young people can apply in their future careers.
"We need a grown-up debate about how to make sure young people get the skills that will get them a job or a place in ongoing education. This hastily assembled report seems to be based more on anecdote than facts."
She said a study last year showed 80 per cent of level 2 BTEC students went on to further study, suggesting progression was more significant than whether they were prepared for work.