An exams chief has said there are "huge question marks" over whether a major part of the skills-led Welsh Baccalaureate qualification will work in practice.
Gareth Pierce, chief executive of the WJEC exam board, accused the Assembly government of being "opportunistic" in the way it introduced the employer-led principal learning qualification last year.
He criticised ministers for not thinking strategically and said there are concerns that it is too big and inflexible for the current post-14 curriculum structure.
Principal learning was developed with universities and employers in England to give Diploma students a taste of a particular work sector and help them learn employment-relevant skills.
Mr Pierce said there are concerns over whether the qualification will work and be widely taken-up following its introduction in Wales.
Four of the qualifications are currently on offer to Welsh Bac students after WJEC teamed up with English exam board Edexcel, and 10 more will be introduced over the next two years.
Mr Pierce has previously warned that principal learning could trap students into a career path too early, and that the qualification would have to be looked at carefully as it was introduced.
His concerns are shared by teaching unions, who said good careers advice is needed in schools and colleges to make sure students aren't forced into making a career choice before they are ready. Heads have also voiced concern that principal learning will eat up teaching time and require extra resources.
At a conference last month, Mr Pierce told delegates that the Government had been "opportunistic" in introducing the qualification, and said he "hoped it wouldn't do anything like that again".
Speaking to TES Cymru after the event, he said it had been "thought through strategically in the context of an overall Assembly government policy for qualifications and the curriculum in Wales".
He added: "One of the difficulties that arises from this is that principal learning is available only in one size - a very large one - so this particular programme of learning is not easily accommodated within 14-16 options structures.
"We did recommend to the Government at the time that they should consider accrediting a smaller version . so that there would be more opportunities for young people to access it."
Brian Lightman, the head of St Cyres School in Penarth, is a leading proponent of the Bac and piloted the qualification at his school.
Although he said there are concerns among heads about the amount of content involved in principal learning and the time it takes to deliver, many are still keen to introduce the qualification.
"It would be a real shame if students in Wales missed out on the opportunity to do these courses," he said. "Take something like engineering - to run a course like that which is valued by employers and universities is something that's very interesting to us as a school and very worthwhile. We would like to see principal learning developing."
An Assembly government spokesman said ministers were "surprised" to hear of Mr Pierce's comments. "We recognise the value that many higher education institutions and, in particular employers, place on principal learning and the benefits of studying these qualifications for students across Wales," he said.
"We recognise how important it is to provide an integrated approach to engage learners and the flexibility of the Welsh Bac provides an excellent framework for the delivery of vocational study options such as principal learning."
Original paper headline: Major part of Welsh Bac `questioned' by exams chief