Fears as Welsh-medium teacher training graduates fall by 46%
Fears have been raised that a lack of new teachers in Welsh-medium schools is putting some subjects at risk.
Figures show the number of students completing secondary teacher training through the medium of Welsh almost halved in the past three years.
One head teacher claimed some subjects, including maths, science and English, were in "crisis".
The Welsh Government said developing a workforce that meets Welsh-medium needs was a priority.
To secure a teacher training place at a Welsh university, students need to achieve a grade B in GCSE English and maths; the same does not apply to English universities.
Gina Morgan, programme director for secondary teacher training at Cardiff Metropolitan University, said the Welsh Government should review the policy, as it put students off studying in Wales.
"One of the reasons, which is a genuine barrier for some of our prospective students, is that they have to have a grade B in English and maths GSCE and several of our applicants don't have that dual profile," she said.
"That really can be enough to put them off from training in Wales. So it's possible we have students, that would like to train with us but for reasons such as the grade B's, they are going over to England and who knows where they will get their jobs."
In 2014-15, 78 students completed secondary teacher training through the medium of Welsh compared to 145 in 2013-14.
Figures obtained by Newyddion9 show 94 students enrolled for the courses for this academic year, but some have dropped out.
Bangor and Aberystwyth universities, which are classed as one centre, has 40 students this year, Trinity St David University has 24, with 30 studying at Cardiff Metropolitan.
Head teacher at Ysgol Tryfan in Bangor, Gwyn Tudur, said failing to attract graduates had been a concern for a number of years.
"A further decrease in numbers this year makes it more serious, it's a crisis in some subjects; maths, science and English," he said.
"We can't attract the best mathematicians and scientists back to schools."
The Welsh Government said developing a teaching workforce that meets Welsh-medium education's needs was a priority over the next five years.
A spokeswoman said: "This means planning to support the development of teachers and assistants, expanding sabbatical schemes for the present workforce and significantly increasing the number of workers in the childcare and early years sectors."