Plans for all new primary teachers to be able to teach Welsh as a second language have been criticised as unfair and unworkable.
Geraint Davies, secretary of teachers' union NASUWT Cymru, said the proposal might even put some people off a teaching career altogether.
The proposal has been included in a consultation from the Assembly government, detailing possible changes to the Qualified Teacher Status standards and requirements for initial teacher-training courses.
It wants to update the standards to bring them in line with those in England, while retaining some specific requirements for Wales.
Among these would be the requirement that all trainees for key stages 1 and 2 must have sufficient understanding of Welsh to teach it as a second language.
Mr Davies said: "I believe the language requirement to be unfair and unworkable. It would put a lot of pressure on trainee teachers who are not as fluent as they would like to be in Welsh. It might turn people against pursuing teaching as a career."
Mr Davies added that the Welsh language requirement could also push teachers across the border to England.
"If it becomes compulsory, teachers trained in Wales will turn their back on Wales and Welsh schools," he said.
Dilwyn Roberts-Young of UCAC, the Welsh-speaking teachers' union, said the proposal would reflect the need of schools in Wales. But he added: "There would need to be sufficient finance and resources in place to ensure its success."
But the National Union of Teachers Cymru accepted the proposal, claiming such a system was already practised in many primary schools.
"It's not as unreasonable as it may sound," said Rhys Williams, NUT Cymru communications officer. "It's simply a way of helping children to have some idea about the Welsh language."
A spokesperson for the Assembly government said trainees would be expected to plan Welsh-language lessons "with support from an experienced colleague and the benefit of extra courses".
Dr Carl Peters, dean of education at Newport University College and spokesperson for the Universities Council for the Education of Teachers, admitted institutions would have to "crank up delivery".
But Janet Pritchard, head of the school of education at Bangor university, said her students, including those from England, had responded well to Welsh lessons.
She said: "We've never had any problem with students not wanting to learn Welsh, but there may be an issue as to whether an institution can sustain the tuition."
The new standards will also include training in Y Cwricwlwm Cymreig, which aims to inform pupils about the Welsh language and Welsh history, and an entry requirement for teachers on Welsh-medium courses of a C grade in Welsh as a first language at GCSE.
In May, it was announced that non-Welsh teaching staff were to be offered three-month sabbaticals to boost their language skills.
Chris Howard, of the National Association of Headteachers Cymru, said: "We have always opposed any unnecessary barriers between the English and Welsh systems, but working in Wales you need some familiarity with Welsh culture and language."
Teachers will also be required to train across two consecutive key stages, although the new foundation phase will be classed as two key stages, to take in early years and KS1. The existing KS2 will also be classed as two stages.
This change has been well-received. The unions see it as a good way for teachers to boost their employability. But there has been some confusion over the way KS2 is to be reclassified.