Welsh is under serious threat in Welsh-medium schools because parents who do not speak the language are not committed to learning it, teachers in the principality have claimed.

The concern about a decline in Welsh spoken in Welsh-medium schools and lessons in anglicised areas surfaced at the National Eisteddfod in Cardiff. Speaking at a debate hosted by the teaching union UCAC, some claimed the Welsh language was not making gains - especially in Cardiff, where just 11 per cent of the population speak Welsh.

Many said teaching in the language was compromised because the academic success of WM schools, rather than the language, was the reason for non- Welsh speaking parents enrolling their children.

Ion Thomas, head of Welsh at Ysgol Gyfun Gwynllyw in Pontypool, said academic success alone was not enough to sustain Welsh-medium schools. “The lingua franca is still English,” he said, speaking through an interpreter. “Our Welsh schools have attracted clientele that haven’t got fire in their belly about the Welsh language.”

Mr Thomas said the choice to attend a Welsh school should be one undertaken along with a “commitment” to the language.

He suggested a promise by non-Welsh-speaking parents to learn the language could be a way forward. Although pupil numbers have been falling in the Welsh capital, demand for Welsh-medium education is increasing among parents who see the schools churning out the best academic results.

At present, just over 5,000 pupils in Cardiff are taught in Welsh.

Strong leadership by heads that ensures classes are divided between all- Welsh and bilingual classes within WM schools was seen as the way forward.

But in schools, the number of pupils from Welsh-speaking backgrounds is declining. In Ysgol Plasmawr in Cardiff, just 13 per cent of pupils come from Welsh-speaking homes. Hywel James from RhAG (Parents for Welsh Medium Education) said parents saw Welsh-medium schools as providing good education and better job opportunities.

Speaking on behalf of UCAC, incoming president Elaine Edwards said Welsh- medium education had to be confident in its mission so that parents and pupils could choose to learn in one language or the other. “We don’t support the position where within the same classroom you have some being taught through the medium of Welsh and some through English,” she said. “In that situation one of the languages will suffer and it will usually be Welsh.”

Iwan Guy, acting director for the National Association of Head Teachers Cymru, believes it is possible to have a school with both English and Welsh streams, but said the issue depended on the strength of the head. “It needs to be very clear which classes are in English and which in Welsh,” he said.

Also speaking at the Eisteddfod, Geraint Rees, headteacher adviser for Cardiff County Council and former head of Ysgol Plasmawr, said Welsh- medium schools in south-east Wales had to be embedded in their community to work well and avoid conflict. But, he said, the fact that every local authority wrote its own Welsh language strategy made the situation difficult.

An Assembly government spokesperson said a national Welsh-medium education strategy was being written and would be drafted by March 2009.