Welsh-medium schools are struggling to implement a new rule limiting the amount of time teachers can cover for absent colleagues, a teaching union has claimed.
The "rarely cover" stipulation came into force in England and Wales last September as part of the 2003 workload deal.
But Welsh-medium union UCAC has called for the Assembly government and schools watchdog Estyn to take action after its members reported a number of problems.
It said many schools, especially those in non Welsh-speaking areas of the country, were finding it difficult to hire cover staff suitably fluent in the language, which was seriously affecting classroom standards.
Its members also reported that the rule was limiting opportunities for extracurricular activities and professional development.
At UCAC's annual conference in Caernarfon last week members backed two motions calling for the government to launch a review into the implementation of rarely cover and for Estyn to look into its effect on classroom standards.
Elaine Edwards, UCAC's general secretary, said: "In some schools rarely cover is working well, but others are struggling through no fault of their own.
"Heads and others in positions of leadership have said they were given no proper training beforehand on how to implement this. Sometimes teachers aren't even aware of what the rules are.
"In some areas they are trying to implement it properly but they are facing money problems and a lack of supply teachers who are fluent in Welsh.
"There hasn't been any extra funding and it has hit some schools very hard indeed; they can't afford to hire fully qualified teachers and are relying instead on teaching assistants and support staff to cover for short- to mid-term absence."
Gwyn Morris, a maths teacher at Ysgol Glan y Mor in Pwllheli, said schools were struggling with rarely cover because it was introduced without proper funding and in addition to existing requirements such as planning, preparation and assessment time.
He said: "If schools want to continue to use qualified teachers, they know they will face far greater costs.
"Once we open the door to supervision and occasional cover for absence by support staff the boundary between supervision cover and filling the timetable becomes blurred and standards are compromised."
However, Estyn and the Assembly government are unlikely to take specific action on the concerns.
A spokeswoman for Estyn said: "When Estyn inspects any school, it is part of the inspection to look at how staff are deployed and used. It is not our intention at present to undertake a survey of this."
An Assembly government spokeswoman said: "The Workforce Agreement Monitoring Group has issued guidance intended to help schools with the processes that they need in order to implement the objective of rarely cover, which came into effect on September 1, 2009."
UCAC's conference was dominated by concerns over education funding, with a special meeting held for members to discuss the situation in their own schools and to share ideas.
The union agreed to set up a committee to look at funding issues affecting the profession and to come up with recommendations to put to the Assembly government.
Welsh language is `evety child's right'
The UCAC teaching union has strongly defended the Assembly government's Welsh-medium education strategy after concerns were raised over its cost.
The strategy, which was launched last week, aims to create a truly bilingual Wales by improving both first- and second-language Welsh teaching.
But, as TES Cymru reported, some in the teaching profession have questioned whether the pound;10 million earmarked to implement the strategy is justified when schools are struggling for funds and resources. However, Elaine Edwards, general secretary of UCAC, said the strategy was "long overdue".
"I think Welsh-medium education is the right of every child in Wales," she said. "It's not fully embedded at the moment; it's actually under threat.
"Unless we have this strategy, there's a danger that local authorities will cut back on post-16 Welsh-medium education and that could lead to an erosion of everything we have achieved over many years.
"When you think of the whole of the education budget in Wales, pound;10 million is a drop in the ocean. This is an investment for the future."
Original paper headline: Welsh-medium schools struggle to implement `rarely cover' rule