Thousands of UK teachers don't have degree in their subject
Tens of thousands of secondary school children are being taught by teachers who do not have a degree related to their subject.
New government figures show that more than one in four maths teachers - around 9,500 in total - do not hold a relevant post-A-level qualification.
This is a slightly higher proportion than last year.
In English, about 8,400 teachers, more than one in five, did not have a higher qualification in the subject.
The situation was worse for physics and some modern languages.
A third of all secondary physics teachers, almost 2,000 people, are teaching youngsters despite not having a degree-level qualification in the science.
And more than half of Spanish teachers are taking classes without a relevant degree, along with a third of those teaching German.
With an average of just over 20 students in a class, it means that tens of thousands of secondary school pupils are being given lessons by staff who could be considered under-qualified in the subject they teach.
Professor Alan Smithers, director of the Centre for Education and Employment Research at Buckingham University, said it was "essential" that teachers had expertise in their subject.
Pupils taught by staff that lack sufficient knowledge risked being turned off, he suggested.
"The absolute essential thing is that a teacher has a good understanding of the subject at the level they are teaching it," he said.
"Our best indicator of that is holding a degree or post-A-level qualification."
Prof Smithers added: "If you have a biologist teaching physics, even at age 11, it may well be that their enthusiasm for physics isn't there, and the child isn't excited by it and moves in another direction.
"It's the understanding and enthusiasm that's important."
He said that schools were still struggling to recruit good staff to teach physics, maths and foreign languages.
"Some of the people without degrees teaching these subjects may have been drawn in because of these shortages," Prof Smithers said.
In other cases, in which lack of experienced teachers was not an issue, such as history, it may be that the way a school's timetable operates means that teachers take classes outside their own subject, he said.
Today's figures show that just over a quarter of history teachers, around 4,500 in total, do not have a relevant degree-level qualification.
The statistics, which give a snapshot for November last year, also reveal that:
:: One in four chemistry teachers do not hold a relevant post-A-level qualification;
:: Neither do a third (32.5%) of geography teachers;
:: Almost two-thirds (62.1%) of information and communications technology (ICT) teachers do not hold a degree-level qualification in their subject;
:: Nor do more than half of religious education teachers (55.3%), over four-fifths (81.2%) of media studies staff and 93.5% of citizenship teachers.
A Department for Education spokeswoman said: "If we want an education system that ranks with the best in the world, we have to attract outstanding people into the profession, and give them excellent training - at the start of - and throughout - their careers."
The government is overhauling teacher training and offering better financial bursaries to top science, maths and languages graduates to encourage them to become teachers, she said.