The National Assembly risks underestimating the real cost of boosting Welsh-medium education in colleges.
Its aim to boost numbers of Welsh speakers could also be hindered by red tape in the post-16 sector, the colleges' organisation Fforwm has warned.
Its comments were in response to the creation of a new unit by the Welsh education and training quango ELWa to promote Welsh in colleges.
The bilingual learning unit will, says ELWa, "revitalise the way in which Welsh is used in the classroom and the workplace, and ensure equal opportunities for those who want to teach, learn or work in Welsh".
Of a total of 229,735 students at FE colleges in Wales in 2000, only 7,994 were studying in Welsh.
The National Assembly, currently reviewing the state of the language, wants more fluent Welsh speakers.
A policy statement produced by its culture and education committees calls for Welsh-medium teaching in further and higher education "to be viewed as part of the mainstream provision".
But research by Fforwm and Coleg Meirion-Dwyfor has found that there are too many funding streams supporting bilingual education in colleges. And they say the Assembly members involved in the review had not initially taken account of the true costs, including staff training and recruitment, translators' services and marketing Welsh to young people.
"It's important that there are opportunities for learners to undertake study through the medium of Welsh, but this requires significant resources as well as a strong lead from the Assembly," said Fforwm.
Launching the bilingual learning unit at the recent National Eisteddfod, ELWa chairman Enid Rowlands said it had been established in response to the Assembly's vision of creating a bilingual country.
"Currently Welsh-medium education is highly fragmented and in the past has received insufficient investment," she said.
"However, we have patches of great strength, in particular in schools, and we need to build on that to ensure there are coherent policies for the whole post-16 agenda."
A specialist group of representatives from business and education has been set up to guide the new unit. And ELWa has said it will work with higher education to ensure there are enough trained teachers and tutors.