Wales' science teacher qualification 'problem' highlighted
Specialist science teachers in secondary schools are in short supply, according to figures seen by BBC Wales.
Figures also show four in 10 of chemistry teachers (43%) and biology teachers (38%) do not have specific qualification in the subjects.
The Welsh Government argued the "vast majority" are "science graduates".
A spokesperson said it was a similar issue elsewhere in the UK.
It comes as an international study of educational performance, Pisa, showed that Welsh students did worse than their counterparts in England, Scotland and Northern Ireland.
In science, Wales' 485 point average in 2016 was 20 points lower than the 505 points in 2006, the 496 points in 2009 and the 491 points scored in 2012.
"I think (the situation) is pretty serious because this has been on going now for five years that we have not seen an improvement," said Bethan Jones, a senior lecturer at Cardiff Metropolitan University who specialises in science.
"It raises questions about the standards of science teaching in Wales."
She said that it was a "huge challenge" to recruit students on to the university's Postgraduate Certificate in Education (PGCE) courses in secondary science subjects, like biology, chemistry and mathematics.
Ms Jones added: "Partly maybe because the Welsh Government do not pay as much for the training bursaries as they do in England."
Owen Hathway, NUT Cymru's policy officer, said: "I think it's a serious situation because it's one we have been talking about for a number of years and yet we fail to address it.
"It's something that puts huge amount of pressure on schools and individual teachers who are there at the moment covering those lessons in sciences and it's a big problem for students in terms of access to that science specialism as well as access to individual support."
A Welsh Government spokesperson said: "As with the other nations of the UK we still do not have enough suitably qualified science teachers, particularly in physics.
"That's why we have introduced financial incentives to attract science graduates into teaching.
"We will also be shortly announcing further details of our new national network of excellence for science and technology that will focus improving teaching and learning of science."