Major reforms of the 14-19 curriculum will fail if standards in work-based learning do not improve, according to Wales's chief inspector of schools.
Susan Lewis will attack training standards among Welsh employers for the third year in a row in her annual report, due next week.
She will say one-third of work venues inspected failed to provide adequate training - particularly in basic skills. Half of them will be re-inspected over the next 20 months because they were so poor.
But in a presentation to Assembly members, she warned the failings in work-based training have wider implications for the Assembly government's plans for reforming the curriculum for school-age teenagers.
Its 14-19 learning pathways proposals envisage a wider choice of vocational and work-based, as well as academic, courses being made available to teenagers. Schools, colleges and trainers are expected to work together to attract and motivate pupils with timetables honed to their job aspirations, with the aim of reducing education drop-out rates and improving qualification levels.
Ms Lewis told the education committee: "The success of the new programme depends on Wales being able to improve its work-based learning sector.
"There is not a good enough work-based framework for the 14-19 learning pathways in place."
Ms Lewis said small and medium-sized businesses in Wales often did not have the time or money to help trainees complete their qualifications. Low completion rates also blight work-based learning, with some trainees having a poor grasp of basic skills in reading and writing.
Some 14-year-olds are already spending half the school week in work placements as alternative curriculums are rolled out. Heads also say they do not have a clue how they will plan or budget for the new 14-19 curriculum.
Neil Foden, head at Ysgol Friars in Bangor, complained to Assembly members:
"We cannot plan for it, let alone cost it."
Janet Ryder, Plaid Cymru's shadow education spokesperson, said the findings were alarming.
"I was shocked to discover that Susan Lewis could only come up with one or two examples of good work-based learning in her report," she added.
"If we are talking about sending 14-year-olds into working environments, and I believe this is the right way for some, then they also need to have personal and social support, which is not there.
"We are talking about children at a vulnerable age, and we need to get this right with training providers who are fully trained themselves."
A report due out next month from deputy education minister Christine Chapman is supposed to give more in-depth guidance on funding and implementing the 14-19 reforms. The Assembly government has set aside pound;41 million up to 2008 to support the introduction of learning pathways. The money is distributed via 22 networks serving each council area in Wales.
It also has a pound;670,000 project in place to support and train providers who deliver key skills training in work-based settings.
And in 2005 it ordered an improvement plan for work-based learning programmes from ELWa, the post-16 education funding agency, following negative reports including some from Estyn inspectors.
ELWa, the National Training Federation for Wales, and fforwm, which represents Wales's FE colleges, said more money and improvement plans meant work-based learning was getting better every year. At last week's meeting of the Assemblys' education and lifelong learning committee, they said the numbers of employers offering work-based training inspected by Estyn was small and not very representative.