Fewer than a quarter of teachers of community languages such as Urdu and Punjabi are qualified to teach their subject, an Ofsted survey has revealed.
More than half of 134 community language teachers questioned said they did not hold qualified teacher status.
Of those who did hold the qualification, half had gained it in a subject other than languages. Training opportunities are insufficient for teachers in a range of minority languages, according to Ofsted's report, Every Language Matters.
Miriam Rosen, director of education at Ofsted, said: "We discovered that when community language teachers were given the opportunity to gain qualified teacher status their teaching improved."
But not all the teachers questioned were convinced of the need for training, especially when they were teaching their first language.
A government review of languages in 2006, chaired by Lord Dearing, described community languages as a national asset and recommended that they should be made more widely available in schools.
However, in 2006-07 there were only 35 trainees nationally studying to teach Arabic, Bengali, Japanese, Mandarin, Punjabi, Turkish and Urdu. No courses exist for training to teach Gujurati, although 1,025 pupils studied this at GCSE level in 2006.
More flexible courses are needed to give all language teachers the chance to gain proper qualifications, Ofsted said.
Pupils can take GCSEs and A-levels in 16 community languages. But the number of GCSE entries overall has fallen nearly 20 per cent, from 26,266 in 2001 to 21,126 in 2006. Over the same period the number of pupils taking Polish went up 77 per cent, from 184 to 325, and those taking Dutch rose 62 per cent, from 195 to 315.