At least one in seven teachers is not trained in the core subject they teach, according to new figures released by the General Teaching Council for Wales.
The data, compiled from the GTCW register, shows that many secondary staff teaching English, maths, Welsh and science in the classroom had initial teacher-training in a different subject. In subjects such as business studies, RE and drama, the number rises to more than 24 per cent.
But the figures also show a trend for younger teachers aged 20-35, to teach the subject they trained in, particularly English and maths.
Hayden Llewellyn, GTCW deputy chief executive, said this contradicted the belief by some that large numbers of teachers in specific subjects will soon retire.
Although teachers do not have to be qualified in a subject to teach it, or to register with the GTCW, there is concern about future shortages in some subject areas.
Recent research by Education Data Surveys showed a 10 per cent drop in postgraduate certificate in education applications in Wales this year in every core subject, including English, maths and science.
However, it is uncertain how many young teachers will remain in their current disciplines throughout their careers, as this is the first year the GTCW has processed the data.
The teaching council acknowledges that some subject areas are easily transferable - for example, a trained chemistry teacher could teach biology or physics. Business studies and economics also cover a lot of similar ground.
Trained teachers can legally teach any subject and age group, including A- level. A further breakdown is expected early in April.